Dressage Basics

I'm hoping to do some dressage with my boy Shea in the future, however he only knows the basics - walk, trot, canter etc. his previous owner was just starting to teach him more complicated stuff before i got him, but they got to busy with work, and had to stop working with him and later sell him.
Generally when i ride i've been working on upward and downward transitions, lateral flexion and just building up his fitness.
What are some exercises or simple things i can train him to do to start off? He knows how to collect, however I don't (well in theory but not in practise) and so want to wait until i'm under intruction to do that with him.
I was thinking i would start him off with something like leg yeilding (and you'll have to excuse my ignorance ) so how would i go about teaching him how to do that?

Hi.
It would be nice if you wanto to start with dressage exercises, and if your horse knows how to collect, shoulder in its a good excercise for horses, because helps to warm long mucles in his body and helps to his balance. You can start in walk, don´t push him too much the firs tesson, only to take a proper incurvation, and how to lisen your legs aid. If you have an arena, you can walk close to the wall, and then, in the central line. Remember to exercise him in both rains , to compensate muscles.
María.