Start out by mixing your colors, especially if the spray bottle you're using isn't clear. These are the three colors I used. I started out with a mix of the light blue and red for the purple, and then later on I added the darker blue to make it, well, blue.
Put water in your spray bottle. Make sure it's a smaller one. You always want to start out with more water in the bottle than you need, because it's easier to add a tad more paint to make it less watery than it is to start over because it's too dark. This is what your paper towel is for. Test out your paint by spraying it after you shake well to mix in the paint.
If you leave a little more water in your paint, it will act more as a dye and thus is better for fabric than it is solid surfaces like window blinds or whatever.
If you make it thicker, it works better for more solid surfaces...like window blinds! (My stupid computer refuses to upload the images I took of my blinds with my spraypaint on them, so you just have to believe me)
Allow your paint to dry.
Viola, you're done. :)