Use an ink suitable for fabric (I usually use Versacraft). Wash the onesies to get rid of any dressing in the fabric (don't use fabric conditioner), press out any wrinkles. Slip a sheet of paper inside the oneise where you're going to stamp so that you have a better surface to stamp on and you ensure there's no bleed through of ink. Then just stamp, allow to dry completely and heat set the ink with your iron to make it permanent (I usually put a piece of kitchen paper or a white handerchief over while I press at this stage, just to be absolutely sure I don't get ink on the base plate of my iron).
HTH!
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