You want to use the "The minimal-movement method", this is where you would filing a tiny bit of metal off both the tip and the shoulder of the key, we can create a bump key that can go just a little bit deeper into the lock. When such a bump key is inserted all the way into the lock, it will be pushed out again a tiny bit by the force of the springs inside the lock, until the pins again rest on the deepest point in the key cuts. We found filing off between 0.25 and 0.5 mm works best, but you may wish to experiment for the best results.
http://www.toool.nl/images/7/75/Bumping.pdf is my source of information.
