Here are the directions to make this shirt from FF's Website:
Corrugated cardboard stars
Red or blue fabric paint
Time needed: About 2 to 3 Hours
1. To make one, first slip sheets of newspaper inside a plain white shirt so that when you print on the fabric, the paint won't bleed through to the other side.
2. Next, cut out several corrugated cardboard stars. Peel away the top layer of each star to expose the ribbed layer. Coat the corrugation with red or blue fabric paint and then firmly and evenly press the painted surface against the shirt. Carefully lift away the star.
3. When you've finished stamping your design, let the paint dry completely and then set it according to the manufacturer's directions.
4. Tip: To make the painting process less messy for kids, glue a cardboard square onto the back of each star for a handle.
My tips or variations: First off, I'd suggest at least one adult for every 2 kids unless your kids are over, say, 8 yrs old. We were having trouble keeping everyone out of the paint and taming the excitement (excitement=big mess). Second, the directions say it is a 2-3 hour project. I'd say we did it in less than 15 minutes. The directions must include drying time. Third, I'd suggest getting the stars ready without telling your kids, so that they don't stand over your shoulder and hover until your done cutting them out. :) Lastly, we added "Jackson Pollack" type splatter to our shirts to make them cuter. We found that when you add the splatter, the imperfect stars done by a 3 year old tend to look LOTS better! I've also made a onsie version of this project when the kids were babies that turned out just as cute!