Once you have your target heart rate calculated pick a cardio activity you enjoy doing. This could be running, biking, swimming, etc. Anything that elevates your heart rate.
Now do this activity for 20 min. maintaining your target heart rate you calculated for yourself. This means stopping every five min. to take your pulse, you can do this by placing your index and middle fingers on your neck or wrist. Count beats for 15 seconds and multiply by 4. If you are below your target heart rate you need to work harder, if you are over you need to slow down.

Record what activity you did, how long you did the activity for, and if you were able to keep your heart rate in your target zone for 20 min.