High knees

Assume a standing position with your feet hip-width apart and your arms down at your sides.

Lift your right knee up to your chest and at the same time raise your left arm bent at the elbow just like in a running motion.

Switch quickly to the opposite leg and arm. The motion is basically running in place with your knees high.
If running in place with high knees is too tasking for you, start with just marching and slowly lifting and alternating your knees.