Great job on the rims! As for the white letters, get a paint brush with a pointed tip and get some acrylic flat white, and only put the tip of the brush in the cap, getting just enough paint for one letter. Then carefully paint the letter. Start with a simple letter, like a F, I, and maybe a H. Work your way through the letters, give yourself time to do each wheel. Try to not let the brush spread and make the letter really wide, keep the point on the brush. If the brush starts to spread, lift the brush off and if you lost the point, clean the brush and get the point again. Try to find a brush you're really comfortable with and practice on a few wheels that have letters. And the beauty of using a acrylic flat white is that you can just rub the paint off under some water and also, you know when it's dry. Best of luck...