By asking our dogs to sit onto a smaller surface area, they are inclined to sit in a “correct” position where both hips & stifles are flexed & weight-bearing underneath the body. I recommend to use an exercise step since you are able to walk your dog up onto the step, it is narrow enough to make the exercise effective, and it is low to the ground in case your dog falls. This exercise also encourages a kick back stand, where the correct muscles are being engaged as oppose to just using the forelimbs.
ADVANCE: Make this exercise harder by asking your dog to perform on an incline. Please use this video as a reference, but try to ensure your dog is not moving forward with their front legs, as this is not challenging the correct muscle groups: