I was never a fan of stamping on fabric. The colors always looked washed out plus you don't have a nice smooth surface to stamp on. So here's my solution...I make t-shirt transfers using Versamark, Embossing Powder and some computer paper. Stamp your image in Versamark Ink on computer paper, sprinkle with embossing powder and heat emboss it. Trim around the image with scissors, place it face down on your fabric and iron. So easy even kids can do it. The embossing powder melts into the fabric, the colors are way more intense then stamping directly with ink plus it's washable and permanent.
Here's a sample using the Gorgeous Grunge stamp set. Note: This method produces a mirror image of your stamp so it isn't suitable for words. The letters for this project were stamped directly onto green fabric that was backed with fabric adhesive, cut out by hand and ironed onto the onsie.