Wash and dry your onesies. DO NOT skip this step. If your shirts shrink after you apply the vinyl, your vinyl will not adhere well.
The next step is to cut out your vinyl. I used my Cricut, but you could go as old school as scissors in cutting yours. Just make sure you cut everything backwards if you use a craft cutter! You can download a PDF of my superhero shapes. The color of the shapes are indicative of the colors they should be cut out in. The gray shapes should be cut from white vinyl. I'd suggest watching some tutorial videos on heat transfer vinyl before you attempt it. It's not hard, but there is a certain order to some very specific steps. It helps to watch it.
Turn your iron on pretty much as hot as it will go and produce zero steam. You don't want any moisture. It'll mess up the adhesion of the vinyl. When the iron is hot, iron the shirt where you plan to adhere your logo.
Lay your vinyl pieces down on the shirt and make sure you know what order to press them (bottom to top layer) and where they will go approximately.Use as much pressure as you can and work on a firm, steady surface.
Press the vinyl layer by layer. Press each layer for a 20 full seconds with the iron. Use as much pressure as you can and work on a firm, steady surface. After the 20 seconds, pull up the iron. You can now gently pull the transfer tape off. Pull the tape off each layer as you go. Use the parchment paper to cover any exposed vinyl. NEVER let your iron come in direct contact with the vinyl.
Double check whether your vinyl is hot or cold peel. If it's cold, you'll need to let it cool before you pull off the transfer tape. If there are any places that stick, re-press before you move to the next layer.