and I couldn't be happier :)
1. You need freezer paper. :) The only place I found it was Wal-mart; next to the tin foil. Cut out a small square/rectangle/circle to cover your idea and onesie; do this three times per onesie. Set two aside until you're ready to iron.
2. Now's the time to put your design on the paper. I usually make my designs in word, publisher, or acrobat, print it out on regular paper and trace it to the freezer paper. (The reason freezer paper can do this is because it has a light adhesive on one side of the paper and it bonds to your project when heat is applied). The adhesive side feels grainy; it's the side that will face your onesies. Trace your design to the SMOOTH side of the paper.
3. Next is the cutting. Use masking tape to keep your design from moving around the cutting surface (I usually use a spare piece of cardboard---diapers' boxes work real well). Think about what areas you want the color of the paint and color of the onesie; cut out the most intricate parts first and remember to save the small pieces needed for white spots (like the middle of an A).
4. Set the iron to cotton/wool; basically you just want to make sure the iron isn't on steam. First, take the second full piece of freezer paper and iron it to the inside of the onesie with the grainy side facing up toward you; this keeps the paint from leaking to the back of the onesie. Check to make sure it's stuck to the onesie.
5. Then, iron your cut-out to front. Iron from the biggest to smallest piece. It takes a couple of seconds for the paper to bond to the fabric. Just make sure it's on there well; be careful not to touch it too soon or you'll get a nice heat shock.
6. Paint. I use an up-and-down motion just in case the paint seeps. Let it set for a 15 20 minutes, then paint another coat.
7. After the paint in fully dry---I usually give it a couple hours---take the stencil off and iron the other full non-cute freezer square on top of the new painted design (this seals it and keeps it from coming off in the wash). Wait for the paper to cool and peel the inside and outside paper off.
Congrats---you've successfully made a new onesie and have become addicted to a cheap and easy way to vamp up kids' clothes--or Christmas stockings!