Walk for 1 minute. Run for 1 minute. Walk for 1 minute. Run for 1 minute. Alternate for 30 minutes.
On an elliptical or bike, the same concept applies. Go for an easy pace for a minute, then go hard for a minute.
To give you perspective, I walk 3 mph and run 8 mph on average. That's the base of the workout. Do what you can do. Maybe you need to walk 2 mph and run 6 mph or 7 mph. Feel free to get creative and mix it up! Sometimes I'll run 9 mph or 10 mph every few sets. Or maybe one day I'll add an incline every few sets. The basic set up is plenty challenging. But the options to do more with it are there when you need it to shake it up and get a change of pace, so to speak!