What should I do after returning to sin, and how can I repent so that God may forgive me?

If you relapse into sin after repentance, then repent to Allah Almighty with a sincere repentance that fulfills its conditions: abstaining from the sin, regretting it, and resolving not to return to it. Allah accepts repentance no matter how great the sin or how often it is committed, as long as the repentance is sincere, and do not despair of His mercy. Increase good deeds, seek forgiveness, engage in remembrance of Allah, associate with the righteous, and stay away from the causes of sin.