Use an adjustable bench. I prefer the benches that have the circular knobs on the side of the bench which allow for precise desired height. For a more budget friendly bench, see this adjustable bench.
Sit close enough for your Right Foot to reach the A pedal, and your Left Foot to reach the D pedal
Sit low enough that your feet are flat on the ground. For the young harpist whose feet may not reach the ground, don’t worry about this! As long as they are sitting at the appropriate height (detailed below), then it doesn’t matter that their feet don’t touch the ground yet.
For pedal harpists, play with a slight heel to get the ball of your foot off the ground and on the pedal. I play with a 1.5″ heel. Always keep the heel of your foot on the ground while you play and adjust pedals. Some harpists prefer a skinny heel, others a thicker heel. Find what you prefer by trial and error. I prefer a thick heel, but first and foremost a sole that is made from real rubber so it grips the pedals well. Try THESE.
Rest the harp equally between both knees and your right shoulder.
Sit high enough so that when you turn your head to the right, your chin clears the soundboard.
The harp should not feel like it is about to fall forward, nor should it feel as if all the weight is on your right shoulder.
We are not all cut from the same cloth, so some harpists will sit slightly closer, higher, etc. to accommodate proper control over the instrument and ease of playing. I am on the short side, at 5′, so I tend to sit closer so I can reach the A pedal and change it with ease. Continue to adjust your settings so you find a comfortable position, especially one that does not lead to pain in your right shoulder.