Step 1: Setting up
*You can destroy the original in the next step, so be sure you already have a (professional)copy of the original if its a critical key*
Step 2: Begin Filing
Begin filing down the key blank to the height of the tallest "mountain" on the original. Try not to scrape any metal off the original, this can destroy the key, derp.
Step 3: File the valleys
Step 4: Testing and fine tuning
When I first tried it, the key didnt work, so I got the file and the original, compared, fine tuned with the file, and tried again. It took a few minutes to get it to work, then another few to get it working as smooth as the original.
I did this just because I bought a few keyblanks for another project but that fell through, and I needed another key, so I thought why not DIY.