I've done a few "at-home" modifications to my car over the years, and this one is one of my favorites (and also the easiest!)
Remove your taillights from the vehicle using screwdrivers or sockets if necessary. The lights usually just twist to one side and pull out. Clean them with wax and grease remover, and let air dry.
Carefully tape off any portion you don't want tinted, in my case, I kept the reverse and turn signals as they were.
Step 1: Homemade Tailight Tinting
Purchase a can of 'VHT Niteshades" and shake very well. The instructions are on the can, but it's basically a translucent black paint. The more you spray, the darker it'll be. I used two light coats so they still illuminate well at night. They will haze up during the process, but I advise that you cover them in two or three coats of a clear (non-yellowing) lacquer when finished to protect them from the elements and keep them nice and glossy.
Wait ten minutes between coats, and apply as directed.
Step 2: Homemade Tailight Tinting
Here's the final product after an hour of curing. In the sun, they are very glossy - my camera hazed them more than reality.
Simply re-install them on the vehicle, make sure the bulbs still light up, and there you go! I advise to apply a light coat of wax (same stuff that you'd use on the car itself) to keep contaminants from eating at the clear coat over time, but otherwise, it's easy as pie.