Protect your clothing and workspace. You may want to spread out a cheap plastic table cloth or a large piece of cardboard to protect the surface you're working on. Large, old t-shirts make great “paint shirts”, or just have them wear clothing that you don’t mind getting a bit of paint on.
To carve your stamps, cut a large potato in half. Use a sharp knife to gently trace a pattern of a mushroom. Then gently cut into the potato, about a ¼ inch or so deep, tracing around the outline of your shape.
Place a small piece of cardboard inside the onesie, separating the two layers of fabric to keep the paint from bleeding through.
Pour your desired colors of paint into small cups or onto a plate. Use a brush to apply a thin layer of paint onto your stamp. Too much paint will result in prints that are goopy and smeared, and too little paint will result in prints that are too light. You will quickly get a feel for how much paint to use. You may want to do a couple test prints on scrap paper to get a feel for it before you start working on your onesie.
Press the paint covered stamp firmly onto the fabric in one swift motion. Try not to wiggle or twist the stamp as you lift it up from the paper, as this can smear your print.
Set aside the onesie to dry. Once fully dried, it’s ready for your little one to wear.