.com/Complete-Training-Horse-Rider-Podhajsky/dp/0879802359Little book with GREAT advice.
Dressage training is just the same as any other training. You can make a great horse, but your horse can act up and be a little dangerous, too, so my following advice might seem a little generic.
Here's the deal--LOTS of people come to realize that horse training takes a LOT LONGER than they thought it would. Therefore, you get lots of advice about find an expert and don't do this or that bc you will RUIN your horse. The ONLY way to ruin your horse is to not train him.
Since your cannot afford to pay somebody to train for you, or to teach you to train, just plan on spending regular training time with our horse and you'll make great progress. Expect the best behavior with every small thing you ask your horse to do. IF you've ever taken like a ballroom dance class the teacher "baby-steps" you through every different dance. Do the same with your horse.
Short (15 minutes sometimes), regular (daily, if possible) sessions make faster progress than infrequent, long sessions.
Watch every free video you can find when you need help. These will help you to recognize disrespect from your horse.
EXPECT your horse to be afraid of some really ridiculous things, like my QH, Buster shied the other day when I snapped a measuring tape near him. I routinely throw hay into his manger from above, sometimes even hitting him in the neck or face and he doesn't care.
With scary stuff you approach and retreat.
PRAISE often for every effort so your horse will trust you.
Watch your horse at liberty. Buster has a lovely, floating trot and will often perform a perfect side-pass when I move him over to clean his stall. You want to know what your horse can naturally do.
If you get him to show at Dressage level tests you can emphasize his strongest skills. Hope this helps! I lent my copy out, but I got a new one Christmas, 2010, and I've been re-reading it.