First step: draw (or trace, I won't judge) your stencil pattern on to the papery side of freezer paper. You can get this at your grocery store-- it has a plastic side and a paper side. Then cut it out. This is where an X-acto knife comes in handy.
Step 2: Iron the stencil, plastic side down, onto the onesie. The plastic will melt and stick to the shirt. Make sure all the edges lay flat, because they can get caught and cause problems. Then dab tons of acrylic paint on the cut out part. Check that you aren't missing any spots.
Step 3: When it's dry (or before, if you're impatient like me), peel up the freezer paper and see how cute it is. Then put it on your kid, because it looks even cuter that way.