Re: 6 foot, 215 pounds , and struggling to do muscle ups.
Focus on keeping your arms in tight against your body as much as possible during the transition, like you are wearing a backpack and you are running your hands from the top of the straps down to the bottom. Don't let your elbows flare out. The one tip that helped me more than anything was to look up at the ceiling directly above you while you are pulling yourself up. When you transition to the dip, throw your head forward and look at the ground directly below your feet.
Look up, kip up. To transition, throw your head forward and run your hands down the straps of the imaginary backpack with your elbows tight against your body.
Try jumping into muscle-ups as well. Experiment with ring height, and try to find a height where you have to jump hard but can still make the dip transition.