Fabric printing is quite easy and can really make your item unique.
First, get some craft foam, I think I used the 10 mm kind, and cut out your shape. It's easier if it's simple like this heart.
Then dump out some acrylic paint.
Add some fabric medium which is found with the acrylic paint.
Mix it up and paint on one side of your shape.
Place it where you want it either on fabric, cut out fabric pieces, or a finished garment.
Press firmly all around the shape.
I found the easiest way to pick up the shape was to lift from one side.
Also, I did mine on a towel to protect my table. The towel makes the finished print splotchy. That was the look I was going for though, and it hid any mistakes. If you want a smooth print, use a flat surface and lots of paint. And practice.
Here is my printed heart onesie I'm working on.
I also tried some metallic paint hearts.
The metallic shows up pretty well even with the fabric medium. The fabric medium also says to let dry for 24 hours and then heat set. I sewed this onesie up while it was "drying" and then ironed it on the hottest setting for 10 seconds in each area with a press cloth.