If one had sexual relations with one of the forbidden Arayot, how can they do full teshuva and tikun? Thank you

Shalom, Thank you for your question. The Torah teaches us that tshevua is always possible. One should start from that certainty – that one is always able to return. On a practical level tshuvah involves firstly a regret of the sin. Then, one should (in prayer) confess their sin to Hashem, and ask His forgiveness. Lastly, one needs to make a firm resolution to distance themselves completely from the sin. In order to atone for the sin (what you call “tikkun”), there are various approaches in the Rabbinic writings. However, it would seem that the most appropriate form in today’s world, is to strengthen oneself in the very area they failed in. That is, to strive to be holy and upright especially in the area of their intimate relations. Keeping the laws of “taharat ha’mishpacha” (if / when you are married); staying away from inappropriate media; keeping the laws of negiah etc. On Yom Kippur you should recall your sin, and ask Hashem for forgiveness (again). May you be blessed with only good.