There are two types of wonder under (or heat ‘n bond). One is more heavy duty–meant for non-sewing projects. The other is lighter, so you can sew over it after it has been applied. I have heard that wonder under is better for the sewing projects, since it’s less likely to gum up. You can purchase it at JoAnn’s, or similar sewing store.
If you’ve been afraid to try it, don’t be! It really is very easy to do, the instructions are clear, and the ideas are endless. If you’re looking for applique tips and patterns, check out THIS post by Char, over at Crap I’ve Made. I used her “1” pattern for mine, and you can find it HERE.
P.S. Can’t you tell how thrilled they were to get their picture taken? :)
P.P.S. I can’t believe they’re 1!! It has gone way too fast. *sigh*