According to Maimonides, the commandment of Teshuba is fulfilled when we recite the Viduy. Viduy consists in verbally articulating our transgressions, admitting our responsibility and feeling regret and shame 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 minister, but right in front of God. Whispering the confession to ourselves.
There is a discussion in the Talmud (Rabbi Yehuda ben Baba vs. Rabbi Aqiba) if one has to specify his wrongdoings or one can just state in general terms that he has acted wrongly. (It is similar to the situation we face when we want to apologize to someone else with whom we often interact: should we apologize mentioning the specifics of our wrongdoing or should we just say a general "I'm sorry"?)
According to the Shulchan Arukh, there is no need to mention every specific sin that one has committed. This leniency aims at not discouraging a person who wants to repent but is not 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 aim to mention in his private confession everything wrong he has done, H. Teshuba 1:1, "and then one shall confess, I have done so and so...".
To follow Maimonides opinion, one will need to apply himself to a deep introspection, exercising greatly his memory, struggling against his own denial, reviewing actions, and perhaps, writing down all misdeeds he can remember.
In this context, the text of the Viduy --the one we say for Selichot for example-- should be seen as a reminder of the subjects we should review, repent for, and hopefully correct.
This challenging 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.
from Prager University