The way I've taught it is by using a horizontal ladder first. I wish I could draw a picture cause I know I'm gonna mess this up a little, but here it goes.....
it's basically a box frame, 10 ft or so long, with 2 x 4's across it. Just basically a ladder, but one that is laying horizontally and more stable, on a frame, with wider rungs. Ours was made all out of wood and the ladder part was about 3 1/2 - 4 feet off the ground, with a step at the front and back for getting to the ladder. It teaches the dog that their legs can move independently, 4 x 4 motion, but without the increased height and instability of a vertical ladder. I'd use another person to help, because you can't let the dog fall while you're teaching it. But that is the way that my dog learned and the easiest for foundation work I've seen. After that, we moved to a stable vertical (angled) ladder that was made out of wood and attached to a wooden platform at the top.
I'm curious to hear other ways to do this...I'm sure there are some out there. This is just the way I was shown for working dogs. Good luck with it!