I've seen this idea a hundred times. My daughter's first grade teacher makes tie-dye shirts this way every year for her students. You can also search "spray paint shirt" on Pinterest or Google and see lots of different examples. There really is nothing to it. First, draw or print off the computer the design you want. I printed off an "O" and "U" and stapled them together to make the OU logo. Then I printed off some hearts to girl it up a little. You can either cut out the shapes and use the remaining paper as a stencil or use the shape to get the reverse look. I did both for this shirt. And you don't have to worry about cutting it out perfectly. It's supposed to look kind of industrial messy.
Next, go somewhere well ventilated but not in the wind. Put some cardboard inside your shirt to make a firm painting surface and to prevent bleed through. Place your cut-outs on your shirt. Here is where gloves or pins to hold the cut-out down would have come in handy. I used my bare fingers instead. Lightly spray your spray paint over the cut-out. Dry according to can instructions. Ours were dry within a few hours. I also washed the shirts before and after. That's it!
Cost comparison : Target $18 x 3 = $54 DIY $4 x 3 = $12 + spray paint $4 = $16
You can see how this ties in to our homeschooling here.