Labfairy is right-use a textile paint. At Joann's and Walmart they have t-shirts that are ready for painting, no special treatment is needed. I got my shirts and paints at Joann's (with a coupon) because everything was in one place and I didn't have to go to different areas of the store. They have instruction sheets at our Joann's altho I think you can download them from their site.

Be sure to use good heavy stabilizer under the shirt to keep it flat! You can even satin stitch the fused item to the stabilizer (do not fuse to the stabilizer) after assembling it on teflon or parchment paper. Then all you will need to do is satin stitch around the edge onto the tee shirt! If you want to try this just let us know!

wow! you must have done this before!

Paint or embroidery. Pics please!

A quick and easy way would be to print it. You can buy iron-on transfer paper sheets specially meant to print on t-shirts, on dark or light ones. At least here in Finland, I'm sure you can in the US, too. Just follow the instructions on the package. Google, or ask in office supply stores. I buy mine from an e-shop where I order my printer paper and ink. By the way, if you do print, make sure to have a MIRRORED image first if you have text on the image, otherwise you'll end up with a mirror image. Here' my Xmas present to myself using this method. I bought the pattern, printed it on the transfer sheet using mirror image setting on the printer, now I'll just iron it on a t-shirt. Hope this gives you an idea especially if you need to make several.
These are Amy Bradley's Quilt Diva pictures from www.erica's.com. Thought I' mention just to avoid confusion. I have bought the pattern, not trying to take credit!