Great trick right here, one of my personal favorites. I've been doing these puppers since I started skating pretty much. I'll try to give my personal insights.
https://youtu.be/EDfDgtZapfY?t=1m8sWhoever said learn them on curbs first was right, that helped me figure out the mechanics of the flat ground one.
Speed is really important, I always found it hard to do ones that looked good until I figured out to go fast. Speed also helps later on when you cradle the board off your thigh, if you pop it right your leg just sucks it up.
Timing the pop and doing it fluid is crucial, if your motions are staggered it looks horrible. A lot of people find them so awkward, because it's a different timing than ollie tricks. The tail hitting the ground should be simultaneous with your foot coming off. It's something that comes with practicing it a lot.
I used to do a lot of weird body leans and shit, I've fine tuned it even more since then. Now I keep my upper body upright and don't really lean any direction(I used to lean really far forward). Having my shoulders more over the board helps keep it under my feet, producing less bails.