Practice, master the single part of whatever dub you are doing and then bounce as high as you can and do it and get comfortable with that. Next do 1.5 flips, if you are going for a dub back do one back flip and then over rotate to your back. Front flip is the opposite, do a complete flip then go to your stomach. (Practicing stomach and back drops helps a lot, also get a throw pad of some sorts) When you feel it send it a litter harder and you should have it. Remember to commit and don't stop halfway through, that's how people get hurt. Once you start a flip do everything you can to finish it cause there's no going back.