I agree with Porcupine, easier when its actually on your head.
The blades and different length guards rely on there being a solid surface to run along and that would usually be your scalp.
But...... clippers do take getting used to using and you could REALLY muck up a piece if you don't know what your doing.

When you first start using them, run the clippers in the direction the hair lays.
So on the back of your head, run them down from the crown to your nape.

This will give you an idea of how much hair they can take off and means you can practice without causing any real problems or cutting away too much hair in one chunk.

Ultimately you run them in the opposite direction to how the hair lays and by reducing the guard size, you create a graduated fade.