Do I make an image in Photoshop, then print onto iron on paper (that I get at... the craft store?), iron onto white onesie, and... voila?! Personalized onesie??
Or is there some secret complicated trick to making them?
That's how you do it... but I have found that you want to print the entire image and have colored backgrounds because the "white space" comes out kind of weird on the shirts... it's clear, but dingy. Does that make sense? Also BE SURE to mirror image your text or it will come out backwards when you iron it on. There will be instructions with the paper.
The thing that I always have trouble with is not waiting long enough before you take the backing paper off. Make sure that you wait until it is cool otherwise you ruin the transfer.
Different manufacturers have slightly different instructions for their transfer paper - some peel hot, some after cooling, so do check the instructions! ...and press firmly whilst ironing too.
One more thing I tend to do - cut off all the unused 'paper' as it can look a little odd without image on it once ironed on.
Are you going to show off your final result to us?? Love to see it...