Do the rims still have the chrome on them? If so you need to remove it either by soaking them in bleach or acetone, or the dye will not stay. Then take your dye (liquid rit dye or the powder packets) and mix it into a pot of simmering water, not boiling because it will wind up splashing when you put your rims in. Then take your rims and suspend them with wire (or just put them in the pot) until you get the desired color ALSO, if you decide to dye your rims black, you cannot go back and dye them red, it can only be a color darker than the last, like from yellow to red is fine
When using the RIT dye, add small amounts to acheive the color you want...you can always go a little darker but never get them lighter. First set i dyed i used blue...dumped the whole packet in the pot and the wheels came out blue....but very, very dark...almost black.