How to do Cheap Easy Screenprinting is here.
I recently wanted to make a print on a blue shirt (continuing the World Cup theme with Italy) and realized I most likely would need to have a white base coat to make the colors strong enough. This turns out to be pretty simple. Just make your initial print encompass everything you will eventually need colored. In my case, this meant everything that needed to be red, green, yellow, or white. This means the original print is pretty vague on the details, however, so it can help to make subtle lines to guide later painting (see the white circle which is lightly outlined in the initial print). Once the white base is down, let that dry (do not iron), then simply hand paint on the next colors. If you only need one color down, you could in theory just re-print on top of the white. I was very happy with how the final colors turned out, and will definitely be doing this again whenever I have a multi-color print or am using dark colored shirts. Hand painting on the screen printing ink works pretty well, however one of my colors (green) was not a very liquid mixture, and did not spread as well as I'd hoped. The red and yellow however were pretty much just like any other acrylic paint. I use paint brushes and sometimes toothpicks for smaller details.