We have just started one of our Morgan mares in western pleasure. She had potential for the English Pleasure Driving/Saddle divisions before she had a bone chip and was on T/O for 1 1/2yrs. Anywho...one of the biggest things you need to establish before you think about slowing/collecting is building the proper back and hind end muscles so the horse can carry themselves in the slower gaits for longer periods of time and have the proper rythem and cadence. Slow and steady is the desired goal, and it is most important to establish the foundation of foward so the horse is capable of doing so. May sound weird, but it works.
For example, when I was working on a QH farm, there was a lovely mare that could go HUS, WP, or Reining. I would always warm her up with a loose rein walk with circles, serpentines, etc to get her loose in her back. Id go into TROT, a nice steady foward moving trot, into frame, still working circles and serpentines, halts, pivots, and turn on the forehand for suppleness. Id really push her foward at the lope the first few laps, then slowly bring her back to a more collected stride, then push her back out again. That was basically the warm up for the WP work...where I would then take a walk break, collect the walk, move into a jog, using circles to check the pace is she got to quick, still did halts, pivots, turn on the forehand, and a lot of transitions between walk/jog then worked on slowing the lope. Even though it is slower, it is still important to be foward moving and not have broken gaits which is a big no no. If you're doing it yourself, use the basic exercises I just described and see where it gets you. But I would advise doing some lessons or talking to trainers to get advice. Best of luck! :)