never seen it done.... might be illegal, but the same as rear lights, although it should be done on the inside of the lens to protect the paint from stone chips and looks nicer.
Basically you want to clean the lenses to the max if u have a dish washer use it.
Then very fine coats on the inside untill consistant.
The problem will be that u should wet and dry between coats, but the lenses have raised patterns on the inside and that makes it impossible!
Thin coats are a must and the overall paint must be thin to let more light through
All said and done the more i think about it the more illegal it is

Might be ok if u only put them on for show every once in a while but other than that i think the police would pull you in a heart beat