The Ten Days of Repentance, as characterized by their name, are designated for repentance. During this time everyone is obligated to repent wholeheartedly before God, blessed is His Name, before the coming of the great and awesome day, of Yom Kippur, as it is said, "Before God you will be cleansed" (Leviticus 16:30). It is also said, "Seek God when He may be found", (Isaiah 55:6) and our Rabbis of blessed memory said, that this refers to the ten days between Rosh Hashanah and Yom Kippur. Therefore, during these days you must examine your conduct and repent of your bad deeds. Sins about which you are in doubt require greater repentance than transgressions of which you are certain, for a person is more regretful when he knows that he committed a sin, than when he is uncertain of it. Therefore, the guilt offering brought in case of doubtful sin must be of greater value than a regular sin offering. During these days You should study more Torah, perform more mitzvohs, give more charity, and spend less time on your business. It was written by Rabbi Moshe Cordovero, of blessed memory, that you should consider these days as if they were Chol Hamoed, by performing only necessary work. Above all, you must set right the wrongs you committed against your fellow man, for which there is no atonement unless you compensate him for any stolen articles or withheld funds¹ and ask him to decide what you should do. It is, however, by no means proper for you to decide on your own, for regarding matters of money, the Yeitzer Harah (evil inclination) will help you find many loopholes. (Mishnah Berurah 606:1) in your possession, and patch things up and placate him so that he will forgive you.

footnotes: ¹ It is also sufficient if you present the case in all honesty before a Rav or Posek

It is proper to observe stringencies during these days that you do not observe the rest of the year, for we also ask of God, blessed is His Name, that He deal with us with exceptional kindness. For example, if during the rest of the year, you eat bread from a non-Jewish bakery, during these days you should eat only bread baked by a Jew; and you should observe similar stringencies.

When reciting Birkas Hamazon, there are some who have the custom of saying Harachaman hu yechadeish [the Compassionate One, may He renew] etc. as we do on Rosh Hashanah.

It is customary not to get married during these days.

On Shabbos Shuvah [the Shabbos during the Ten Days of Repentance] an eminent person should be called up for maftir.² merely states that a youth should not be called up for Maftir on this Shabbos. (Mishnah Berurah 602:11)

footnotes: ² Mishnah Berurah

Some people have the custom not to sanctify the moon before the conclusion of Yom Kippur, for then everybody is joyful, whereas before Yom Kippur they are apprehensive about the outcome of their judgment Others maintain that, on the contrary, it is better to sanctify the moon before Yom Kippur in order to add to our merits with this mitzvah. It all depends on the prevailing custom. In a region where esrogim, lulavim and hadassim are for sale, it is the custom of pious men to purchase them ahead of time, so that during these days this precious mitzvah may be added to their merits.