In the form they usually just ask for yearly incom. You state it there. And when they ask for an employer you mention "selbstständigerwerbend" and mention the name of your company (if you have a website, add the site).
If they do ask you for more, get a reference from your current landlord.
I wouldn't worry too much in advance. Do the trial and error thing, apply for a flat - you might just get it (saved a lot of worrying) or you'll find out what they ask for