These are different variations that can be practiced depending on your strength. The core and arms should be engaged, shoulders stabilised and the pelvis tucked in. Once you feel strong, try Side Plank (Vasisthasana), either one knee on the floor, both feet on the floor or one leg extended to the ceiling or Reverse Side Plank (Viparita Vasisthasana). Remember that the arms are not designed to carry our whole body weight so engage your core and legs to make the pose easier.