How can one repent from major sins committed out of ignorance of their prohibition, and is seeking forgiveness and refraining from them sufficient for repentance?

Whoever commits a sin out of ignorance is excused for it, and there is no sin upon him. As for repentance, it has six conditions: Sincerity to Allah, ceasing the sin, regretting it, resolving not to return to it, that it be at the time of its acceptance (before the throes of death or the sun rising from its west), and restoring rights to their owners if they pertain to the rights of people. If the sin is against Allah's rights, then the first five conditions suffice, and Allah forgives all sins for whoever repents with sincere repentance. It is obligatory to gain understanding in religion to avoid falling into prohibitions.