Be sure to put a piece of cardboard inside the pillowcase so the paint doesn't bleed through to the other side. Use a foam brush to apply paint to the bottom of your stamp (use fabric paint, or mix fabric medium into regular paint).
Place the stamp on the fabric, press firmly, and pull straight up. Repeat until you're satisfied with the pattern. Let the paint dry (I always pull out my hair dryer at this point to speed it up) then do the other side.
My girls had a great time making their own pillowcases t0 take on vacation! Hope you'll try it out and have fun creating with your kids!