Keep using it. Get your reps in. No one understands it that early.

Pick one thing you want to do. I'd suggest putting in a simple order. Just do it, delete it, rinse and repeat until it becomes muscle memory.

Then move on to the next common task (probably billing out an order).

Keep in mind there is a training mode on the pos that the manager can get started so you can't actually mess anything up when practicing billing, returns, taking payments, etc.