How do you do a flat black paint job?

It's almost impossible to do with a brush. Get a good spray can (Krylon or similar) and prep your material well, then spray it in a shop or garage where temp. is around 75 deg. Start with very light coats, overpainting gives a different texture.

A good spray can? What the hell? There is no such thing as a good spray can. Get a paint gun and get some flat black paint from the auto paint supply store and learn.