Interval training is simple - implement short bursts of more intense cardio exercise during your routine workout. Here's one example of what you can do on a treadmill to implement interval training, but you can also watch the short video below as I explain and demonstrate each of these elements. 

1 - Start walking on the treadmill at a pace that you feel you can maintain throughout your workout. You should be able to talk without sounding out of breath, but singing may not be doable. 

2 - After a 3-5 minute warmup, increase the speed of the treadmill (or increase its incline) to push yourself a little harder for 30 seconds. After 30 seconds, return the treadmill speed and incline to your starting position and go for two or three minutes. 

3 - During your recovery period (which is that 2-3 minute break from the more intense elements of your workout) your heart rate will gradually slow down to roughly the same level you were at before your burst. 

4 - After your recovery period, go for another 30-second burst of intensity, then return back to your initial treadmill settings. 

5 - Continue implementing those short bursts of increased exercise throughout your 15-20 minute workout. 

6 - Once you have completed your final burst and recovery period, slow the treadmill down gradually over the next few minutes to help your body cool down. NOTE: The warm-up and cool-down periods are very important for your body during a workout, so don't skip them - you're body will remind you later that you missed them.