The easiest way is to use a degree grinder. With 3mm cutters, use good tape to keep the wheel from rotating and then work through the slot. However, this can cause the cutter to break off during short quick movements.
For 6mm cutters, we go into the locking channel of the cylinder with a small screwdriver to tighten the washer of the protective fitting. Then we use the 6mm cutter and mill the rotating disc down (away from the screwdriver) into two halves. Now we remove the screwdriver, rotate the disc 180° and put the screwdriver back in. Now we repeat the milling process and destroy the 2nd side. After that, the disc should be cut into two parts and can be removed from the fitting. Now the core is exposed for further milling work.