How do i quit after being on f.m.l.a?

You need to return to work (for at least one day), if possible. This will completely stop any claim they could make that you committed job abandonment by not returning from your FMLA leave. If you simply cannot return, submit a signed statement (I would actually send it through certified mail, so you will have a record in case it gets 'lost'), noting the reason you are unable to return to work.