Here are a few of Rebbe Nachman’s teachings on tshuva, from his book, “Likutei Etzot” (“Advice”) translated by Rabbi Aryeh Kaplan. Elsewhere on this site, you can find Rebbe Nachman’s famous “Tikun Klalli” over the spilling of semen in vain.

Even if a person should fall to the lowest level, to the lowest pit of hell, G-d forbid, he must still not despair in any way, regardless of his condition. He should remain firm and search for G-d, pleading with Him and begging Him to help in whatever way he can. Even in the lowest pit of hell, G-d is present, and even from there it is possible to be attached to Him. As King David said, “If I make hell my bed, behold, You are there” (Psalms, 139:8).
*
No matter how greatly a person may have sinned, as long as he is still called by the name “Israel,” he is still a Jew in spite of his sins, and the radiance of the root of his soul can be transmitted to him wherever he may be by means of the study of Torah. Then he will return to G-d.
*
Even when those who were very distant from holiness are aroused to return to the light of the Torah, they are still far away and they may experience tremendous obstacles. It takes enormous effort for them to strop themselves of their “filthy garments” (Zecharia, 3:4). These “filthy garments” are as difficult a barrier as a river which is impossible to cross. If you want to draw closer to G-d, do not be discouraged if you find yourself confronted by all kinds of obstacles. This is inevitable, because all of these obstacles stem from the “filthy garments” – the sins of the past. It takes great efforts to strip off these “filthy garments” and throw them aside. At times the experience is very bitter. But in the end, all obstacles and barriers which stand in the way of holiness will disappear.
*
Tshuva (repentance) means to return a thing to the place from which it was taken – to restore it and return it to its root. Now wisdom is the root of everything.  This is why you must guard your mind and your wisdom against alien ideologies and extraneous thoughts, not to mention evil temptations. The reason for all the sins and transgressions which people commit, and all their shortcomings, is that their thoughts are impure. They are not careful about guarding their thoughts and not overstepping the boundaries of holiness. When a person protects his thoughts and his wisdom, he can remedy everything and return to G-d.
*
If a person feels genuine pain for having sinned – if he “circumcises the foreskin of his heart,” and feels true contrition for what he did, and returns to G-d with all his heart – then the hearts of all the drops of semen that left him – his actual children and the drops that were wasted and spilled in a foreign domain, G-d forbid – will also be circumcised in the “foreskins of their hearts.” They too will feel the pain of their degradation. They will be aroused and rise with a thunderous noise and return to G-d.
*
The only way to attain complete tshuva is by passing through all the places one had been before tshuva – and obviously this will vary for each individual, depending upon what he went through in the earlier part of his life. When he passes through them and encounters the very same temptations that he experienced before, he must turn his head aside and control his inclination without repeating what he did in the past. This is the essence of tshuva. There is no other way.
*
If you want to accomplish complete tshuva, you should make it a habit to recite Psalms. They are a great help to tshuva. Even a person who feels no enthusiasm at all for tshuva can experience an awakening if he recites the Psalms. No matter where you are, you can always find yourself in the Psalms you are reciting. This will arouse you to tshuva and help you find the gate of repentance which is suited to your soul. You will be able to open the gate and attain perfect repentance, and then G-d will return to you and show you His love. This is why during the month of Elul and the Ten Days of Repentance, all Jews recite Psalms. But this is something one should do throughout the year in order to accomplish tshuva.
*
Tshuva helps for every sin, even the most serious sin of all – the deliberate emission of semen in vain, or other forms of grave sexual immorality. When the Zohar states that tshuva does not help in the case of a person who wastes his seed, the meaning is not what it appears on the surface. The truth is that as our Sages have said that, “There is nothing that stands in the way of tshuva” (Jerusalem Talmud, Peah 1:1 and Sanhedrin 10:1; Rambam, Laws of Tshuva, 3:14; Zohar Shemot 106a: Tosefot on Baba Metzia 58b.) But perfect tshuva can only be obtained with the help of the true Tzaddikim.
*
A person who experiences a nighttime emission should recite the following ten Psalms on the same day: Psalms 16, 32, 41, 42, 59, 77, 90, 105, 137, 150. One who recites these Psalms on the same day need have no further fears about the harm such an emission can cause. Any damage will certainly be repaired through doing this. It is a very great tikun.