Hard to describe but I'll try.
Normally you ride "straight up" - not leaning forward or back (if you believe you could have horse disappear and still be standing then you're in the proper position).
For forward (driving aide) you push your hips forward (lower back but waist up remains where it was) - some people refer to it as rolling the hips forward.
For HH you push your weight into your stirrups and stop "following" with your seat, sitting "heavy" for a second.
Experiment with those two aides and see what you get. Don't forget while driving to soften elbows to allow forward movement, and while HH'ing lock elbows on waist during HH to restrict movement.
Dressage is for Trainers!