With your dog standing square, firstly lift up one foreleg & hold flexed under the body, encouraging your dog not to put any weight through the limb & your hand during the exercise. Next, lift the contralateral hindlimb (diagonal limb) & hold flexed under the body. It is sometimes helpful to bend the toes under to discourage weight-bearing.
To begin with, try to have them stand in this position for 5-10 seconds & gradually increase to 30 seconds as their confidence improves.
INCREASE: Increase the challenge by working on a soft surface.