I'm autonoma and I do the paperwork myself with my Spanish husband. If you don't speak Spanish I would recommend going to a gestoria. If you do speak Spanish it means going to the local tax office (go there first) and then to the INEM office. Ask them which forms you need and go through the papers with the people there. If you get the right person then it's not too painful a process. If the person is a typical fucionario and is very unhelpful just stick to your guns until you get the info you need. If it gets too complicated you can always get someone to do it for you.
By the way you can go on and off the register as many times as you want so if you've got a lean month coming up you can "darte de baja". Just make sure you go back on again when you need to.