Using the largest number guard you think you will want for the sides, buzz all the way up to the part in your hair, basically where the sides meet the top.
I go with a 3 for this step.
Stepping down one number at a time, make passes around the sides of your head not going any higher than a couple inches below the last level you shaved. This results in a fade.
Going around the back of your head is tricky for obvious reasons. Having 2 mirrors handy helps a lot but I find that using my other hand to feel first, then act as a guide for the buzzer works very well. Doing this, I've been able to get a fairly straight and even cut around the back.
With the sides all squared away, wet the top of your hair. Feel around to become familiar with the differing lengths. We are going to reproduce the same relative lengths but on a much smaller scale.
This step is best done with scissors using your fingers to guide the cut. Alternatively, you can run the clippers against your fingers but I find scissors to be much less awkward.
Generally, your hair should be longest in the front and "fade" left to right and to the back. The exception would be if you have a specific part or coif in your hair that calls for longer or shorter hair.