You're local tackle shop should do it for you if you buy the line from them. They'll put down some mono backing, and work it out based on the class of the braid you select (and give you advice on this too). They'll also set the drag correctly, so I reckon it's worth getting them to do it.
Leave it to the pro's I say, rather than save a couple of bucks on the line and stuff it up.
Some tackle shops will also do it for a small charge (~$10), even if you don't buy the reel or line from them...
Good luck,
Mike