I did one for DD's bday last year. I was really nervous that I would ruin it, but it came out great!
The most important thing to remember is to check if you need to print a BACKWARD or RIGHT-FACING image. The transfer paper (available in office supply and craft stores) will give detailed instructions about which way is correct for their paper. Pay attention!
When you go to iron it on, the directions advise against using an ironing board. I used my smooth glass-surface electric stovetop and was pleased with the results.
Also, keep in mind that you will probably be able to wear the shirts only once or twice. The do-it-yourself iron-ons are not known to survive many machine washing and drying cycles.