Everything that works at "slow" starts with collection and balance. Work on getting your horse to work from behind and engage their hind end. Once you have a balanced collected horse then you can focus on moving slower. It is very hard for a horse to slow down if they aren't balanced and collected.
Get your horse collected and balanced at the walk, working trot, and canter. Then once you have that down pat then work at setting a slower pace. You will have to use all of your aids for this that you do to get collection. Slow them up with your hands softly and at the same time push them up from behind. Your goal is to get them to carry their back on a level line. In a western horse we want the hips to do a little bit of a weeble wobble type of motion. Once they get that going then you can slowly work at using less aid to get the same affect.
Hope that helps