How would one even go about doing magic for a restaurant?
You need to approach the restaurant and sell yourself to them. Convince them that you can help them make money! (Lots of threads on this with some Google searching!) However there are a couple of other ways. One is to know the manager/owner. As most circumstances, who you know is so important!
I got started by being a busser (cleaning tables), and doing magic for servers tables when I was asked to (Doing magic helps get the server bigger tips!). Often if an order was messed up, or wait times were long, a server would have me go over and perform for that to lighten their mood. Or a server would have a larger group and would have me help them keep that table having a great time (again for better tips!)
Then I became a server there and did magic for my own tables primarily (as well as helping out other servers upon their request). The owners heard about me doing that and were initially concerned. Then they let me show them what I did and they quickly changed their tune and hand me come in for some shifts to just do magic. It certainly wasn't the quickest way, but I was just learning magic at the time.
Restaurant magic is a great way to learn and get some chops!