Here's a quick guide to tinting light with a spray can type tint. It costs about $30 NZD for a can but that would do at least 4 sets of lights. Also it is illegal to have any alterations like this on lights in my country and although I haven't had any trouble as of yet, be careful.
Step 1: Remove Lights
For my car I just had to pop the boot and there were two screws underneath it for each tail light. Undo the screws and the light slides out a small way. Reach in behind and undo all the bulbs, They were a stiff quarter turn anti clockwise to undo. Remember where each bulb went so you can put them back correctly!
Step 2: Mask Up Lights
Use masking tape that isn't to sticky so as to leave residue on the lights and cover up all the parts you don't want tinted.
Step 3: Paint Them
Put them somewhere that doesn't matter if it gets oversprayed. Follow the directions on the can for flash off time between coats. Paint until darkened to your liking.
Step 4: Before and After
This is the second time I have darkened mine Because It fades slightly after a while.