These practices are to help you understand posture, control, and stance. They're done on the beach with a board with the fins removed. You can understand foot placement when riding on a SUP with a centerboard by placing your feet on the board's centerline.
Notice your footing on the board. Are you too close to the nose or tail or closer to the edge? See how the board responds to your feet. Too close to the nose? You'll feel pressure to fall forward. Too close to the tail? Your board will want to lift from the nose. You want equal heel-to-toe pressure when riding on a SUP.
If riding a SUP without a centerboard, you will need to review supplemental instructions on applying the correct heel pressure needed to edge your way upwind. This also requires fine-tuning your stance and weight distribution.
If you want to move on your board, do this shimmy slide. Move your feet heel-toe sideways or forward without lifting your feet. This will help you have your weight distributed when moving positions. Try to switch between the different footing and weight distributions. For example, favoring your front or back foot or by standing on one leg.
Repeat all exercises with goofy and regular stances.
Regular: Right foot is closer to the nose.
Goofy: Left foot is closer to the nose.