painting rims is easy. I would suggest changing brake pads to ceramic ones as they create way less dust. So clean the rims first, I do mean clean. they can't be too clean.
Sand them with sandpaper, I suggest a light 120 grit, then a 150 then a 180 grit. You can skip some grits but honestly that is the part that will give you the best looks so... don't skip a grit. You don't have to pound the day lites out of it either. You want your primer to stick and stick well so once its scratched real good that is enough. Clean the rims again.
Then take a deck of ordinary playing cards, stuff a single card between the rim and tire, overlap with another card and stick it in and go all the way around. This will stop the tire from being painted. Then a quality primer, feel the rim- IF rough then sand to a finer grit, clean when primer is dry and then put your paint on. Use quality rim paint so the job lasts a long time. You will be proud of your work.