First I use #2 guide all over...front to back, then back to front, then side to side. Use two mirrors to see it all; you will probably find that you need several passes in each direction to get it all even, at least at first. Next I switch to #1 guide to get just the sides of my head, going "against the grain". Blending the edges comes with practice.
For the back of the neck I take a piece of masking tape, and put it so that the bottom edge is where I want the bottom of my hairline to be. While facing the mirror, I hold both ends so that they are level, then I tape the ends to my ears. Trim the back of the neck using just the clippers (no guide.)
Finally I use the masking tape all over my head like a tackcloth to help remove any remaining loose hair fragments that didn't get brushed off, then wash. Pretty easy once you get used to it. Hope this helps.
My procedure is very similar, except I start with a #4 on top and taper down to a #2 on the sides/bottom back.
It’s very easy once you learn the lay of your head and where to switch or just adjust the length of the guide you’re on.
I figured out the masking tape trick on my own too (except use the blue tape, ouch!). It was only a couple of years ago I fought out other people underwent the same epiphany.