This article talks about straightness (aligning te horse so that he can use his back and hind legs beneath himself): How To Straighten The Horse
Video with Jane Savoie demonstrating shoulder alignment during a circle: Dressage Trainer Jane Savoie Discusses Straightness - YouTube
You can try this one, but mostly it just lists some signs the rider can use to identify that their horse is not straight: Straightness: A Tale of Two Horses - by Thomas Ritter | ClassicalDressage.com
Lateral flexion instead of verticle (headset): Dressage Exercises For Young Horses | HorsePaddock.com
Basically, don't worry about the head right now. You want to work on teaching your horse to bend, and that starts with moving away from your leg. For instance, on that 20 meter circle you were doing, if your horse understands moving away from your leg then you can start leg yielding out on the circle. Then leg yielding in (decreasing the circle size). This helps the horse by increasing his flexibility, teaching him how to step under his body with his inner hind leg more, and helps to align the horse (make him straighter) so that he can start using his back. When he begins to do that, his head naturally begins to come more verticle and his neck arches. The "headset" you see in dressage is a byproduct of engaging the hind end. It doesn't work the other way around. You can't set the head and get engagement. The more you crank their head down the stiffer they get and the more on the forehand they get.
In my opinion, if you want to learn something, then research the heck out of it on your own. After that, find the most qualified person you can to teach you. Settling for mediocre means that the best you will be is mediocre. I'm watching a very talented young friend go through this currently. She loves her trainer dearly, but after a year of dedicated riding she's still stuck on 20m circles and a headset. While her best friend (on a far less talented horse) beats her repeatedly at shows. It's sad to watch... Posted via Mobile Device