Category I - How To Do It:
Choose a cardio activity and get to any equipment you need in order to do it. For example, you can run on a treadmill or track, use an exercise bike, or jump rope. Warm up with light activity (such as a jog, or light pedaling if you're on the bike) for five minutes. Now up your intensity so that you're working at near maximum effort for 15 seconds. If you're on a treadmill, for instance, this would be a sprint.
Next, slow down or stop (depending on the activity) and figure out your heart rate: Find your pulse next to your Adam's apple and count the beats for six seconds. Multiply that number by 10 to find your heart rate in beats per minute. When your heart rate drops to 120 beats per minute, perform another intense bout of your cardio activity. Repeat the process for nine total intervals. Time your rest periods and try to perform your intervals with less rest in each succeeding workout.