There are two main ways to do a heel raise:
Standing calf raise: Stand with your feet shoulder-width apart and your toes pointing forward. Slowly raise your heels up off the ground as high as you can, keeping your toes flexed. Hold for a second at the top, then slowly lower your heels back down to the ground. Repeat 10-15 times.
Single-leg calf raise: Stand on one leg and raise your heel off the ground as high as you can, keeping your toes flexed. Hold for a second at the top, then slowly lower your heel back down to the ground. Repeat 10-15 times on each leg.