I have stamped on onsies before my son was born. I had contacted SU about what I needed to do...
1. Wash artical of clothing- DO NOT USE FABRIC SOFTNER (Liquid or sheet)
2. Iron out any wrinkles
3. Get a peice of card board or a another heavy item to put inbetween front and back. (The ink will bleed through to the other side)
4 Hold clothing TIGHT and Stamp using a BOLD Stamp (something that doesn't have a lot of detail) and CRAFT INK.
5. Heat set with heat tool.
6. Wait another 24 hours and then wash again.
Just keep in mind that over time no matter what you use whether it be fabric paint, transfers, or craft ink it all fades. Also what I found most helpful when stamping on something that was washed A LOT (onsies) dark ink stays longer- Blues, Reds, Greens (otherwise BOLD colors). Yellows- tend to fade VERY FAST!!
HTH and have fun!!!