Prewash onesies and dry without using fabric softener.

Cut large squares of fabric to make the designs. Paint Mod Podge on the back and let dry.

At this point you can cut designs freehand or run the fabric through a die cutter. If you use a die cutter, I recommend using a new mat to keep the fabric held down while it's cutting.

Apply a generous amount of Mod Podge onto the back of the appliqué. Be sure to get a lot on the edges.

Place an appliqué onto the onesie and press firmly. Be sure to press from the center out to the sides to ensure there are no bubbles.

Smooth with your fingers. If you get any extra decoupage around the sides, wipe it up quickly or dab with some cold water to remove.

Let the onesies dry overnight.

If after the first washing you notice that any of the appliqués are peeling up, simply seal them with additional Mod Podge and let dry.