I put polka dots on one of Z's shirts yesterday. The idea came to me after I realized the shirt had a big, ugly stain on it, and it wasn't coming out (no matter how many of those pinterest laundry tricks I tried). So I decided to cover it up with some paint.
silver acrylic paint
30mm foam stencil brush (something like this or this)
Place the cardboard inside the shirt, so the paint won't seep through to the back. Make sure the shirt is lying flat. Use the stencil brush to make your dots. The key here is to make sure you have enough paint on your brush to make a complete circle. And when you place the dot on the shirt, push the brush down and tilt the handle around slightly to make sure you get a complete circle on the shirt. You may want to practice a few times. There is really no rhyme or reason to where I placed the dots. It was all random. If you don't like random, you could always break out the ruler.
Now I just need to make one for myself.