I got the long sleeved onesie/babygro at M&S here in England. They are thicker than other onesies so they can be worn as part of an outfit, rather than just under things.
1. Draw a design in light pencil, chalk, disappearing ink, or anything that can be washed out easily.
2. Turn your onesie inside out. Take a pretty piece of fabric and place it with the right side against the onesie front (wrong side up). Pin into place.
3. Stitch around design using embroidery thread. It doesn't have to be perfect, just follow your drawing and knot off at the end.
4. Take little, sharp scissors and cut the tshirt on the inside line of the stitching, but leave pretty fabric underneath.
You are done!!
This is a great project to make while you are watching the last episode of LOST that your husband refuses to delete off the DVR.