Begin with a plain or mostly plain t-shirt. Stick the adhesive letters, numbers, and what not onto the t-shirt the way you want it to look.
Now that you're done with your design, you put some carboard inside the shirt, in case the paint leaks through.
When you've finished that, you can begin spray-painting over the letters, however you desire it to be!
I suggest spray-painting it outdoors because the fumes are smelly!
When you are satisfied with the look of it, let the paint dry.
After the paint is dried, and you have nearly gone crazy anticipating the outcome of your shirt, you can start peeling the stickers off! =D
After you've peeled all the stickers off, and you absolutely love your shirt, flaunt it!!! =D