After the Mickey Mouse shirt bleach pen debacle, I've learned to embrace fabric paint and freezer paper stencils. It's so much more forgiving.
I'm totally on the chevron bandwagon.
Print out your image at the correct size. You probably don't want to use too detailed an image. You have to cut them manually, unless you have a Silhouette to do the cutting for you. Tape this to a surface you won't mind cutting. I used this. You could also use a larger cutting board.
Place a piece of freezer paper (not wax paper, not parchment paper, just freezer paper) over the printed paper and tape down. Make sure the shiny/waxy side is down. Trace around the areas that you'd like to cut. With a more detailed image this can be a bit confusing. Whatever you cut out will be what will be painted on your onesie. Is that a negative image? I over think these things and then my mind wanders to thoughts like "Is Roquefort from Aristocats the same voice actor as Winnie the Pooh?"It might help to shade in the areas that you want cut so you don't mix it up halfway through cutting. Using your X-Acto Knife (I have this one. I'm glad I spent the few extra dollars for the more comfortable option.) Cut out your stencil.
Dump all of the water out of your iron and let it heat up. Put the onesie on your ironing board (I used a wooden cutting board instead) and iron out any wrinkles. When the fabric is cool to the touch, lay out your pieces of freezer paper on the onesie - shiny side down. Once they're in the right place, iron them down. It helps to pick up and move the iron vs. sliding it. Make sure the edges are sealed. The heat of the iron melts that shiny waxy stuff onto your shirt. Don't worry, it peels off very easily.
Keep going, just a few more steps.
For some unknown reason, they still make puffy fabric paint. Don't buy that. There are a lot of elements from the 80's coming back, but I hope to God that puff paint isn't one of them. This is the kind that I bought. It came with some small brushes, but I used a foam dauber brush that I got in a $1 bin at Wal-Mart.
Once the fabric cools, slide a piece of cardboard between the front and back of the shirt. Paint over the exposed areas of your stencil. This may require a few coats to get to the right look that you want. A few thinner coats seems to hold up better than one thick coat.
Let the paint dry completely and peel off the freezer paper. Be careful as you peel around the edges.
Heat your dry iron back up. Put a pillowcase or another piece of fabric over the painted area. Run the iron over the covered painted area to heat set the paint.
I like that I was able to make Disney stuff for the kids that was a bit more understated than the OMG DISNEY PRINCESSES GLITTER stuff that I've found in the stores.