Not that this shirt isn't dirt cheap at Forever 21. Or that the zipper the back isn't awesome...but it's a triangle. And I have some paint! Want to make your own too? keep reading.
A good old white tee
Smooth out your tee as best as possible. Wash it first though! Or you could use one you've washed a ton and it might have a small yellow stain on the front. Like me. Your choice.
Put something inside the shirt to keep the paint from bleeding to the back-cardboard from a cereal box works great.
Make your triangle! I wanted mine as big as possible without the triangle really creeping onto my sides or back. Start at the top where your v is already-that will keep it more centered. And a word about the washi tape...it's great for this! I was worried because it's rice paper, but that stuff worked perfectly. Prettiest painter's tape ever.
Start painting! (you put something inside the shirt, right?) Swirl, dab, paint away. I was extra careful around the edges-I slowly dragged my sponge edge along all the edges to make sure there was a nice crisp line.
Now depending on your fabric paint directions, you may have to wait like me. Mine said to let dry 4 hours and then wash 72 hours later. Be sure to gently flatten it out again as best you can. Obviously that's not entirely possible (mine is pretty wrinkled) but it'll help keep it from cracking. I pulled the tape off after a few hours just to be sure I didn't need to do any touching up.
Feel free to ask me any questions in the comments! I'd love to see what you make! Wouldn't this be super on a onesie?