Step 3: Cut the top
On my second attempt I made a cut from the edge towards the stem as kind of stress relief and used several light pushes downward to cut the outer ring off. If you want to stay completely on the save side, cut the outside of the keys with heavy scissors.
Anyway, repeat this about 100 times. Scrape off any plastic from the top and sand it lightly. Make sure you keep these keys as well as all the screws and all the parts you take off save in boxes or jars. I like to use e.g. old pill bottles, empty margarine/soft cheese cups or a glass jar.