According to Maimonides, the commandment of Teshuba is fulfilled when we recite the Viduy. Viduy means 'confession,' and it consists on the articulation of our transgressions, admitting our responsibility and feeling regret for the wrong things we have done.
The Viduy/confession is done privately. We do not disclose our sins in front of other people or a rabbi, but right in front of God. Whispering the confession to ourselves.
There is a discussion of the Talmud if at the recitation of the Viduy one has to specify his wrongdoings (Rabbi Yehuda ben Baba) or one can just state in general terms that he has acted wrongly (Rabbi Akiba).
According to the Shulchan Arukh, there is no need to mention every specific sin that one has committed during the last year. This leniency aims at not discouraging a person who wants to repent, but does not find himself capable (or courageous enough) of recalling the specifics of his bad behavior.
Maimonides, however, rules like the first opinion and indicates that one has to mention in his private confession, everything wrong he has done.
To follow Maimonides opinion, one will need to apply himself to a deep introspection, exercising greatly his memory, struggling against his own denial, reviewing last year's actions, and perhaps, writing a list of all misdeeds he can remember.
Additionally, the text of the Viduy that we have in the Machzorim --the one we say for Selichot for example-- should be seen as a reminder of the subjects one has to review, repent for, and hopefully correct.
As we have said before, all this spiritual/ethical activity cannot be done overnight. We dedicate to it forty days, from the beginning of Elul until Yom Kippur: the day we devote ourselves entirely and exclusively to Teshuba/Viduy.
Shabbat ends in NYC: 8:15 PM
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"Confession and Redemption"
by Rabbi Zev Leff (Aish.com)