It's really simple:
- Pick an exercise (e.g. sprints - running or bike, squats, jumping jacks, dumbell presses, you name it)
- Warm up
- Do as many reps of your chosen exercise as you can in 20 seconds
- Rest for 10 seconds
- Repeat steps 3 and 4 eight times.
I guarantee you'll be knackered on your eighth set. You can tailor the sets to your level of fitness by choosing an exercise that you're comfortable with.
I'm doing these in conjuction with lots of base fitness stuff (walking, cycling and swimming) and in conjunction with the Primal Eating plan.