It is important to do this exercise along with the front plank. This side plank exercise will strengthen the core muscles along your sides.
First make sure the elbow is on the floor directly under the shoulder. Raise the hips off the ground, balancing on the forearm, elbow, and feet. It is very important to keep the body in a straight line, including the head.
Hold this position for as long as possible, working your way up to one minute. If this is too difficult for you, it can be modified by balancing on the knees rather than the feet (as shown in the lower image).
This exercise should be done at least 3-4 times per week.