This will explain the process I went through to copy a key with a vice, a file and patience.

Step 1: Setting Up

You will need to clamp the original and blank together in the vice in a way that you can use the original as a pattern for the duplicate. Be sure to have the entire key lined up perfectly and tighten the vice.

*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

This is where patience comes in handy.
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

Now start filing the valleys, be sure to keep checking that the keys are lined up during this part. I just used the file at an angle, but if you have a smaller file that would make this alot easier. You maybe able to use a Dremel® tool, but I dont have one.

Step 4: Testing and Fine Tuning

The copy I made was for my front door.
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.