Pretty much all there is to it. Just drop them an email and ask if they are interested. You will find that a lot of the sites on pages 2+ are not really monetized all that well so finding someone to sell you some link space is pretty easy. Now quite often you will not get a response or sometimes they are not interested so you just move on and contact the other sites. I usually do them in batches of five at a time rather than just one.
What percentage of site owners get back to you and are good with selling you a link or ad space?