First, read the permissions and pass the little test, linked at the bottom of the screen. Then just find the cards you own with missing scans, scan your cards onto your computer, and upload, per site guidelines. Billy Kingsley also has a useful guide linked in his forum signature, that will help you get high quality scans.
I'm going to reevaluate how I collect after the new year. It's just getting way too expensive for the new stuff. Sometimes I just want to buy a pack, not a whole box or even blaster.