The second best way, or if you tow a variety of trailers, especially borrowed ones that you can't just go swapping out bulbs in willy nilly, is use the XM381 converter box.
install is pretty easy. Only downside is the trailer lights will only work when your headlights are on (has to do with how the converter box gets/uses power)
or build one yourself; it's really just a 24-12V converter, several relays, wiring, trailer plug, and a box.
EDIT: and just be warned, if the "trailer hitch" you just installed is one of the pinball type (flat piece of steel that bolts where the pintle is and a 2" receiver welded to the bottom and not much else) then you won't be towing very much weight. Those things bend pretty easily if you tow more than a few hundred pounds or use more than a 2" drop for the hitch itself; too much leverage on it.
If you got a real hitch like the one from Federal or BlackDog, then you're good to go, and maybe consider a trailer brake controller if you're towing more than 3k lbs (or whatever the legal amount is in your state to require brakes)
