This is a fun project, because they comes together so easily. I have made onesies, but I have also made cute Spring t-shirts.
I ordered my printable fabric online a year ago, but recently I have seen it in craft stores. You can find fun and free images from many sources, the ones I used were from here (baby books), here (flowers), and here (
Once it is ready, iron fusible web to back of printer fabric and cut out image using pinking shears to make it look like a stamp. Iron to onesie or t-shirt. Hand or machine sew around edge of image and you're done.