I just re-keyed a set of K bags for a friend and its REALLY simple. Don't pay anyone else to do it. I think I did the whole set in less than 30 minutes.
I'll try to explain. Unlatch the lock (open position) and put in the existing key that works. Using a paper clip, depress the end retainer tang that holds the lock cylinder into the plastic latch housing (I curled the end of a paper clip and it worked fine). Its a blind operation, but after you do it once it all becomes clear. The lock cylinder should slide right out.
Remove the old key, insert the ignition key, and file down the pins that stick out until they are flush with the cylinder housing. The material is soft, and you don't need a lot of pressure to grind away a bunch of material. Then wipe away the filings, grease the outside of the lock cylinder housing, and re-insert into the latch housing. It only goes in one way.
If this operation sounds too difficult, do what I do: practice on other people's bikes! :)