From the time you start until the 5 minute mark: exercise steadily and not too hard, to bring your heart rate up to about From minute 5 until minute 10: exercise a little harder, to bring your heart rate to then for 1 minute exercise REALLY HARD, to bring your HR to for about a minute (as long as you can!) then slow down until your heart rate drops to when it hits or below, speed up again and bring it back to for a minute then slow back down to then back to then slow down but LESS, to then back to , then slow down, to for a minute then slower, spend a few minutes with your heart rate about before stopping.
The total is about 22-25 minutes, depending how long it takes to get your heart rate up, and how long it takes your heart rate to slow down. In the beginning it may take your HR some time to slow down. Just slow to a light jog (or elliptical equivalent) til it comes down, then crank up your speed again.
Of course, if at any time you feel chest pain or feel like you are dizzy or you might faint, SLOW DOWN and then stop!
Also: It is possible that you might find the above too easy. These are estimates based on a formula using age/ resting heartrate; the gold standard is to use VO2Max testing to find out what your heartrate is when you enter anaerobic metabolism. This estimate works pretty well though, but keep in mind that the idea is: when you are at the max heartrate, you really should feel like you can't go any more, and can only continue at that rate for maybe a minute.
If you feel like this isn't giving enough of a workout, just let me know; we can increase everything a bit.
I don't tell anyone to push their HR past 180. At that point I'd say if you don't feel maximal exertion, stay at that rate for a bit longer, say up to 2 mins.