Self binding is a fun and economical way to bind. My reference says to leave twice the desired finished binding width plus 1/4" for a seam allowance. For example, for a 1/2" finished binding, cut the backing so it extends 1-1/4" beyond the seam line.
Here's a tutorial that is helpful:
http://www.whatididatschooltoday.com...ory_a_qui.html