When you're starting out, make sure you practice placing your back foot over your rail fins, and planting your front foot right where your chest was when you were paddling.... which should be right between your hands when you pop up. Work on nailing your foot placement first, and everything else will be easy to figure out. Once your begin to progress, you'll want to start nudging your back foot back to eek out every drop of performance your board can deliver. But for starters, right over the front fins. That's the most neutral, stable place for your foot when learning.
Pumping... as mentioned... is critical for hpsb surfing. Gotta keep driving off your fins/back foot to generate speed. After that, you'll be looking to use more rail. Some people say "learn to use your rail first," but I disagree. Hpsbs are about using your thruster or quad fin array first, then your bottom contours and rails.
But surfing off your back foot alone is not enough... you gotta figure out where on the wave to turn/pump/drive. That takes time in the water... no way to cut corners on that one. Just standing there trimming like you do on a longboard or funboard will not help speed up the learning curve, and isn't why you shortboard to begin with.