It's pretty easy, the state's Dept. of Revenue has all the information online.

http://www.revenue.wi.gov/faqs/pcs/seller.html

I have a mobile sellers permit for doing local events (I don't have a store) and that only cost me $15. It renews for free, automatically, every year. I have to file one extra piece of paperwork with my state/federal income tax forms, but otherwise that's it.