When Our Sages Speak
In the first section dealing with the laws of Pesach,  the Alter Rebbe writes: It has become customary in these later generations for the [local Rabbinic] sage to expound upon the laws of Pesach on the preceding Shabbos, unless that Shabbos is Pesach eve,  and upon the laws of Sukkos on Shabbos Shuvah (the Shabbos of Repentance).
The essence of the matter is to speak about and teach G-d's ways and give instruction concerning the deed which must be performed.
This custom has its source in the customs of the Maharil, and is also cited by the Bach and the Magen Avraham. There is, however, a slight difference between the wording chosen by the Alter Rebbe and that chosen by the Magen Avraham.
The Magen Avraham  states: "The Maharil would also expound on the laws of Sukkos on Shabbos Shuvah."
By adding the word "also," the Magen Avraham implies that, on Shabbos Shuvah, the Maharil would not only expound upon the laws of Yom Kippur, he would also speak of the laws of Sukkos. 
The Alter Rebbe, by contrast, does not mention the word "also" which alludes to Yom Kippur, and speaks only of the laws of Sukkos. Why does he omit mention of the need to speak of the laws of Yom Kippur?
Another question arises: Shabbos Shuvah is not always the Shabbos directly before Sukkos. In many years, there is another Shabbos between Yom Kippur and Sukkos.
On the surface, the exposition of the laws of Sukkos should take place on the Shabbos directly preceding the holiday. In particular, this is true with regard to the Alter Rebbe's ruling, according to which Yom Kippur is not mentioned. Why should the laws of Sukkos be mentioned on Shabbos Shuvah, even when it is not the Shabbos which directly precedes Sukkos?
Also, there is a difficulty with the wording used by Alter Rebbe afterwards: Why does he distinguish between "G-d's ways" and "the deed which must be performed"? What is meant by these terms? 
It is possible to explain as follows: The Alter Rebbe alludes to the fact that on these two Shabbosos, our Rabbis should discuss not only the laws of Pesach and Sukkos, but also concepts that relate to our Divine service and observance of the mitzvos at large. This is alluded to by the term "G-d's ways."
Similarly, the Alter Rebbe does not mentioned Yom Kippur, because the essential laws of Yom Kippur do not require special elaboration. They are included in the "ways of G-d" which are taught on Shabbos HaGadol and Shabbos Shuvah.
To cite a parallel: It is not customary for the Rabbis to speak on the Shabbos which precedes Shavuos, because "Shavuos does not have distinct laws of its own. All the prohibitions and leniencies which we observe are also observed on Pesach and Sukkos." 
Body and Soul
The expression "ways of G-d" is worthy of explanation.
A "way" is a medium to get from one place to another.
The ultimate goal is not the way itself, but the destination. Nevertheless, it is impossible to reach the destination without the "way."
The ultimate purpose of the entire creation is that the Jewish people should observe mitzvos in this material world.
Nevertheless, "a mitzvah without [the proper] intent is like a body without a soul."  And therefore in order that one should observe the mitzvos in a proper way, the love and fear of G-d is necessary, for they infuse vitality into the observance of the mitzvos.  They are the "ways of G-d" which lead to the ultimate goal of observing the mitzvos.
It is true that "deed is most essential." 
If someone would have in mind all the intentions associated with a mitzvah, but fail to actually observe the mitzvah, he would be transgressing G-d's will.
When, by contrast, he observes the mitzvah, even if he lacks the proper intent, he will have fulfilled the essence of the mitzvah, thus carrying out G-d's will. 
Nevertheless, the observance of the mitzvah should be with energy and vitality, and this is possible only through love and fear.
One might protest: What's wrong if one fulfills the mitzvah without vitality, merely to satisfy one's obligation? Although one's deed is "like a body without a soul," still despite the fact that "the soul" is lacking, "the body" is intact, and seemingly, that's the essence of the matter.
In reply, it must be explained that when a person observes mitzvos merely to fulfill his obligation, or as a habitual act, the routine in which he has been trained, there will ultimately be a lack in his actual observance of the mitzvah.
At the outset, he will observe the mitzvos without hiddurim, fastidious care.  For when a mitzvah is performed without vitality, merely to fulfill one's obligation, this fastidious care will be lacking.
Moreover, this approach will inevitably lead to a pattern of spiritual descent. For if a person does not invest his energy and vitality in the Torah and its mitzvos, it will seek other outlets and find expression in matters opposed to these spiritual purposes. And this sets in motion a spiral downward.
Perhaps at the outset, the person's observance will remain sound, for afterall, he seeks to fulfill his obligations. And therefore, he will control his feelings and desires and do what is required of him. But afterwards, since his desire and his energy is focused on matters outside the sphere of holiness, he will seek - and find, for "a bribe [and particularly a bribe of self-love] blinds" -  loopholes, and ultimately he will become involved in forbidden matters. And then, "one sin leads to another,"  and he will continue downward, transgressing prohibitions for which he cannot find a loophole. Ultimately, he will not oppose his feelings and desires for forbidden matters at all, and will follow them without remorse.
For these reasons, a person's observance must be charged with the energy and vitality which stem from love and fear. Therefore they are called "the ways of G-d," for they are the paths that lead to perfect observance of the mitzvos.
Fashioning G-d's Dwelling
Moreover, even when the actual observance of the mitzvos is unaffected, when a mitzvah is performed without vitality, like "a soul without a body," it does not fulfill G-d's will. G-d's will is that the mitzvos be permeated by vibrancy, that they should be "living mitzvos."
To explain: The mitzvos were given with the intent of "refining the created beings."  This implies that the person performing the mitzvah will be refined and be brought into connection with G-d.
Ultimately, this connection should permeate all of a person's different potentials, and the innermost depths of his soul. Accordingly, if a person observes the mitzvos only with the intent to fulfill his obligation, the mitzvos will effect only his power of deed; that is the only potential which will be come connected to G-d.
This runs contrary to G-d's will, for G-d desires that every aspect of a person's character should be connected to Him. This is achieved when a person invests all of his energies in the observance of the mitzvos. 
Although the Divine intent is that all of a person's potentials, including his conscious powers, be connected with G-d, and the connection the conscious powers share is higher than that shared by the power of deed, nevertheless, this connection, the expression of love and fear of G-d, should not be considered as an independent purpose. Instead, the purpose of love and fear is to introduce vitality into the actual observance of mitzvos. 
For the ultimate intent is the establishment of a dwelling for G-d in this lowly material world.  This is achieved through using the lowest powers we possess, performing mitzvos with the power of deed. Nevertheless, the consummation of G-d's dwelling comes about when a person observes the mitzvos with all of his potentials. 
On this basis, we can appreciate the intent of the description of love and fear as "the ways of G-d."
The ultimate purpose is the actual observance of the mitzvos; this is what establishes a connection to G-d's essence. Nevertheless, "the ways of G-d " are the mediums which bring that connection with G-d's essence from a hidden state into revelation. Our understanding, love, and fear of G-d bring out openly the connection to G-d established through our deeds.
The above also enables us to understand why the Alter Rebbe speaks of "the ways of G-d," using the plural term.
With regard to the observance of the mitzvos, by contrast, he uses the singular form "the deed which must be performed."
The distinction can be explained as follows:
Although there are 613 different mitzvos, they all share a fundamental commonalty; they all have the same purpose: dedicating one's power of deed to G-d. Therefore, the singular term is in place.
When, by contrast, one speaks of love and fear, there are many different levels, for everyone serves G-d according to his personal level. Hence, the plural is employed.
In general, there are two types of "ways" (or it can be said that a single way serves two purposes):  one of ascent and one of descent (in analogy, one from the field to the king's palace, and one from the king's palace to the field).
In general, this represents the difference between the Divine service of the month of Nissan, and the Divine service of the month of Tishrei.
Nissan expresses the motif of drawing G-dliness down within our world, while Tishrei gives expression to man's potential for ascent as explained in other sources. 
Therefore, the custom is for our Rabbis to speak twice a year, on the Shabbos before Pesach, and on Shabbos Shuvah, for these two Shabbosos give expression to these two motifs: the Shabbos before Pesach to the motif of drawing down G-dliness, and Shabbos Shuvah to man's ascent.
These two addresses thus include the Divine service of the entire year: the address of Shabbos HaGadol includes the Divine service of the summer months, and the address of Shabbos Shuvah, the Divine service of the winter months.
- (Back to text) Shulchan Aruch HaRav 429:1.
- (Back to text) In which case the talk is given on the preceding Shabbos. 3. 429:1.
- (Back to text) This interpretation is offered by the Machtzis HaShekel. 5. Significantly, the Magen Avraham uses slightly different wording, stating: "to speak about and teach G-d's ways, giving instruction concerning the deed which must be performed." By omitting the word "and," he implies that "the deed which must be performed" refers to "the ways of G-d" mentioned previously. The Alter Rebbe, by contrast, adds the word "and" indicating that these are two different subjects.
- (Back to text) Shulchan Aruch HaRav 429:2.
- (Back to text) See Likkutei Torah of the AriZal, Parshas Eikev; Sheloh, Masechtes Tamid, Amud Tefillah, Inyan Chanukah (p. 249b); Tanya, ch. 38.
- (Back to text) See Tanya, chs. 4 and 38; Kuntres HaAvodah, ch. 2, p. 15.
- (Back to text) Cf. Avos 1:16.
- (Back to text) See the sichah to Parshas Mishpatim in this series where this concept is explained.
- (Back to text) It must be emphasized that these hiddurim relate to the essence of the mitzvah. For mitzvos are G-d's will, and will is a simple expression of the soul that knows no division. The difference between the fundamental requirement of the mitzvah and its observance b'hiddur are relevant only with regard to the concepts of reward and punishment. (See Sichos Chag HaShavuos, 5693, Likkutei Dibburim, p. 1540. See also Shulchan Aruch HaRav 481:1.)
- (Back to text) Devarim 16:19.
- (Back to text) Avos 4:2.
- (Back to text) Bereishis Rabbah 44:1.
- (Back to text) See Tanya, ch. 40, which describes love and fear as "wings" which elevate the observance of the mitzvos and which explains that "the purpose of love is Divine service motivated by love."
- (Back to text) See Tanya, ch. 36.
- (Back to text) To explain this concept within the analogy of a dwelling: There are two aspects to a dwelling: a) that the entire essence is found in the dwelling, b) that in one's dwelling, the essence is revealed. The latter dimension is contributed by love and fear which introduce vitality into our observance of the mitzvos.
- (Back to text) See Biurei HaZohar, the beginning of Parshas Pinchas and the maamar entitled Havayah Yechatu, 5689, ch. 6.
- (Back to text) See the Zohar, Vol. II, p. 186a which explains that Nissan is called the month of spring, "aviv" in Hebrew. This name illustrates a progression of the letters from the beginning onward, while the name Tishrei begins from the end of the alphabet. Similarly, the Zohar (ibid., p. 51b) states that the progressive order of "aviv" points to the influence of chesed, "kindness," while the reverse order of Tishrei, points to the influence of gevurah, "might." See also similar concepts in the Likkutei Torah of the AriZal, Parshas Vayeitzei, entry Sheva kochvei lechas.
(Note, however, other sources, e.g., Taamei HaMitzvos, Parshas Bo as quoted in Ateres Rosh, in the beginning of the maamar dealing with Asaras Yimei HaTeshuvah; Pri Etz Chayim, Shaar Rosh HaShanah, ch. 4; Shaar HaKavanos, Inyan Rosh HaShanah, p. 4; Nahar Shalom at the conclusion of Etz Chayim, in the Warsaw printing.)
See the discussion of this subject in Kehilas Yaakov, Maareches Yud Beis Chadashim.