During the period of Aseres Yimei Teshuvah every person is to perform an accounting and introspection of his actions [Cheshbon Hanefesh] and repent from his ways. [One is to spend some time in seclusion each day contemplating his good and bad traits and devising a plan in bettering himself.]
A questionable sin: A questionable sin requires greater Teshuvah than a definite sin. The reason for this is because by a definite sin a person has total regret with all of his heart. However by a questionable sin one tells himself that perhaps he did not sin at all and he thus does not regret it with all of his heart which causes its punishment to remain upon him. It is for this reason that the Karban Asham which came for a questionable sin cost more than the Karban Chatas which came as a result of a definite sin.
The auspiciousness of Teshuvah during these days:
Even though Teshuvah is accepted by Hashem throughout the year, during the Aseres Yimei Teshuvah between Rosh Hashanah and Yom Kippur ones Teshuvah is of better quality and is immediately accepted before G-d, as it says in the verse “Search Hashem when He is found, call to Him when He is close.” [The Teshuvah of these days comes from a deeper source within the soul, and is hence more graciously accepted Above.]
Like Chol Hamoed:
The days between Rosh Hashanah and Yom Kippur are considered like Chol Hamoed. They are thus forbidden in Melacha and one may only learn Torah, repent and seclude himself during this time.
Each day is Mitaken the day of that week for the previous year:
Sifrei Mussar and Chassidus:
One is to spend his time especially during these days learning subjects that discuss betterment of one’s character and spirit. The Rosh states that one is to learn the Sefer Igeres Hateshuvah of Rabbeinu Yonah. Others state one is to learn Sefer Hayashar of Rabbeinu Tam.
The roots of sin:
The Kabalists write that the roots of all sins are Gaavah, depression and forbidden speech.
Matters that require amending:
One is to especially fix those matters that people stumble on constantly and don’t give any heed that they involve a sin. These include: Arayos [Forbidden contact or speech with another woman]; Stealing, which includes withholding payment from a worker. Lashon Hara, which includes all derogatory speech about his friend, even if true. Prayer, to add in concentration. Shabbos, to keep all the detailed laws and guard one’s mouth from speech forbidden on Shabbos. To set times for Torah study.
All new resolutions are to be taken upon oneself without making a vow. Thus one is to state that he is resolving to do it “Bli Neder”.
Diminishing words of speech:
Throughout Aseres Yimei Teshuvah the Rebbe Rashab would speak very little.
 603/1; Rama 603/1; Mateh Moshe 829; Elya Raba 602/3
 Kaf Hachaim 603/11
 Admur ibid; Rama ibid; Rabbeinu Yona Brachos; See Likkutei Sichos 3 Tzav p. 209 [Hebrew]
 Rambam Hilchos Teshuvah Chapter 2 Halacha 6
 Siddur Haarizal in name of Reishis Chochmah in name of Ramak in name of Rabbeinu Yona; Hisvadyos 1983 1/40
 Siddur Arizal; Kaf Hachaim 603/20; See Hisvadyos 1984 1/33
 See Hisvadyos 1984 1/33 that although in the Siddur Arizal it only states that the week of Aseres Yimei Teshuvah rectifies each day of the previous year it is understood also that it serves as a precedent for every weekday of the coming year and the fact it fixes also the days of the previous year is a greater novelty.
 Birkeiy Yosef 603/4
 Mateh Moshe 832; Kaf Hachaim 603/21
 Shaar Hakedusha of Rav Chaim Vital; Birkeiy Yosef 603/3; Kaf Hachaim 603/7; See Tanya chapter 1 that Gaava is the Yesod Haeish; Forbidden speech is the Yesod of air and depression is from the Yesod of earth.
 Machazik Bracha 603/1; Kaf Hachaim 603/11
 Machazik Bracha 603/2; Kaf Hachaim 603/12; Shaareiy Halacha Uminhag 5/69; See Beis Yosef 603; Aruch Hashulchan 603/2; Piskeiy Teshuvos 603/1
 Sefer Hasichos 1944 p. 3