Scuff the lens with 600, tape off the areas I don't want tinted, light mist of Bulldog, coat with the tint, if I'm doing red I have had the best results with Testors Candy Apple Red from a hobby store, if I am smoking I mix just a little black base coat into the clear about 10:1, then the number of coats you do determine how red/dark it will be, let it flash, tack cloth, then remove the masking, a light mist of bulldog to adhere to the parts you had masked, and bury it in a 2k clear.
If you are doing a set of tails with amber color that you want to get rid of (or turn the clear reverse portion red) I like to tape off everything but that section, spray a few coats, then unmask the rest of the red, and do a coat or two over the whole thing to even it out.
In progress.
Finished result, before polishing.