If you are making your own stamp, one eraser is big enough for two square-ish stamps, or you could do one longer stamp. Draw your image right onto the eraser. Keep it simple because the paint won't pick up really detailed images like ink would. Cut out half the depth of the stamp around your image.
For the paint, you can use any color regular acrylic paint. Add one part fabric painting medium to two parts acrylic paint. The fabric medium can be found at both craft and fabric stores.
I used a foam brush to coat the stamp for each star. Also. I poured way too much paint. Live and learn.
If your fabric is really thin, you'll want to put a layer of freezer paper or wax paper between to prevent seepage to the other side. My pattern wasn't perfect. Some stars are closer than others, but I think that's part of the charm of a DIY.
Once you do both sides (allow each to dry) check the side seams to see if you need to fill in any stamps. I added a few to keep the pattern relatively even. Allow to dry completely dry for 24-48 hours and then press for 30 seconds to set the paint and it should be wash-proof!
And on the back, I broke the pattern just a little bit, to remind her how much she is loved ;)