Find somewhere with rolling terrain including some significant climbs. From here, the job is simple-ski stride the uphill portions with good ski striding technique. If you don't know what this means, find someone to demonstrate it for you. Basically you will be imitating diagonal stride without skis-very cool. On the flats and downhills just run normally.
This workout can be performed at various intensites. We typically do this workout in Zone 3, and occasionally in Zone 4. This means aiming for heartrates in the target zone on the uphill portions, and about 1 zone lower on the downhills and flats. A zone 3 workout, for example, would mean trying to be in zone 3 on the uphills, and zone 2 on the downhills and flats. If you have no idea what I am talking about when I refer to zones, the following article explains them pretty well: www.nordicskiracer.com/Training/2004/CampZones.asp
We generally do this workout about once a week during the summer months, for an amount of time varying between 15 and 45 minutes total time.
Be sure to include thorough warm-up and cool-down runs of at least 10-15 minutes each.
It is often helpful to have someone watch you (not including those passers-by who will stop to wonder, gawk, and poke fun at this extremely weird thing you are doing) to make sure ou are not cheating by just running up the hills and lazily and sloppily planting your poles in the ground.