It’s simple if you know how. Register Blue is putting together a how-to guide. In the meantime, here are a bunch of other guides, from the League of Women Voters, Rock the Vote, and others.
http://lwvnj.org/images/resources/lwv_voter_broch.pdf
http://www.everydaycitizen.com/organize-a-voter-registratio…
https://www.rockthevote.com/…/field/voter-registration-how-…
http://www.aauw.org/re…/organize-a-voter-registration-drive/
We’ll update here when the RegBlue one is done. Stay tuned!