I started by using Amazon. I blogged about birding in Florida (i.e. places I went, birds both common and not so common, good spots for different seasons, etc.) and after I started to get a following became an Amazon Associate. I would put links to birding guides I used, binoculars, cameras, gear and anything else that would be useful to birders. I easily made anywhere from $600-$800 a month. It was a lot of work though and I gave it up a couple years ago because I just got burned out.