The beauty of yoga is that the practice is always growing. Once you learn a pose, you can continue to build on it with challenging variations.
Side plank pose (Vasisthasana) is a terrific way to build arm, wrist, shoulder, core and leg strength. As a balancing pose, it challenges you to develop greater stability and confidence while strengthening major muscle groups.
You can move into side plank from plank pose. You want to glue your legs together and roll onto the outer edge of your right foot, stacking your left leg and foot on top. Then press down through the inner edge of your right hand and inhale your left arm up, stacking the shoulders and hips.
Once you feel strong in this posture, you can move on to the full expression of vasisthanasana that challenges your arms, core and legs.
To get into the advanced pose, bring the foot of your upper leg onto the inner thigh, taking care the hips are lifted high and pushing through the lower leg to keep it strong.
Then exhale and bend your upper leg, bringing the thigh towards the chest. Hook your big toe with your fingers, inhale and extend your leg straight upwards.