This project was strongly inspired by DaveM's Stream Cheap, but has 12 buttons instead of 8. For information about assembly and wiring you should head to his website.

Since there are function keys up to F24, you can assign each button to its dedicated key.
Here is a modified code that does just that.
Additional 4 switches should be soldered to pins 10, 16, 14 and 15 on Arduino (for this code to work).