This method doesn't require much skill or practice to get right. If you do make a mistake, well, if you have enough hair on your head to wear a mohican, it will grow out soon, so don't sweat it.
Step 1: Equipment
Electric clippers are much more effective at removing hair than scissors, especially when you can't see what you are doing.
If you want to shave all the way down to your scalp, you will need an electric razor. The trimmer should be a sturdy slide-out design, and NOT a flip-out trimmer that is too flimsy or too awkward to use when you can't see what you are doing.
You will want to do this in front of a mirror! An additional hand mirror will allow you to check your work in the back, but you won't be able to hang onto it while you are trimming. The beauty of this method is that you are doing it mostly by feel.
Lastly, you need hair. As long as your middle of your hairline is somewhat forward of the top of your head, you can even have a huge forehead like mine.