I was looking at the Gate of Intentions by Rav Chaim Vital about Sukot, and it mentions there the importance of repentance on the night of Hoshana Raba. [The 7th day of Sukot after midnight] That is when the memo is given to the angels that was sealed on Yom Kipur. There it says there is still hope to repent before the angel of judgment gets the memo. But how to repent or on what to repent is the question. The most obvious answer is to learn Musar [books of ethics].  This is because every person is on their own path and can't know what they are doing wrong except by seeing what others do wrong. כל הפוסל במומו פוסל, all who cancel cancel with their own defect. SO only by seeing what is wrong with others, can one see what is wrong with himself.