Start with a 30 min warmup where the last 5 min or so are done at a faster pace. The warmup is also a good oppotunity to find a slight uphill where one skate is not too difficult. Once completed make your way to the identified hill and mark a 50 m zone with poles or waterbottles. Make sure it is possible to see any oncoming cyclists or rollerskiers to prevent collisions.
The middle part of this workout usually lasts about 20 minutes but it should not exceed 30 minutes. Athletes are to sprint the marked zone over and over again until the 20 min period is done. Make sure youngsters or really competitive elders know this is not a race and that feeding rivalries will be counter productive. If athletes show signs of slowing down before the end of the workout it is better to stop before the time is up. At any time a coach or competent fellow skier should observe technique and make constructive suggestions if necessary. This workout is when it is useful to try different things. If the course has a turn in it, try different approaches that make the change in direction most efficient. It is also a great opportunity to develop the different 'gears' for every technique. Hard slow 1 skate and fast 1 skate are essentially the same but there are minor differences in body movement that completely change the resulting movement. Skiers can get to know what trade off (speed vs effort required) every gear of their techniques have and choose them in consequence during a race.
End with at least 30 min of slow skiing for the cool down.
This workout doesn't leave you with the impression of having worked out very hard yet so many improvements can take place in these sessions. Most of my technique and balance improvements this year have taken place during these workouts. They are obviously very good to improve top speed used in sprinting. This is where a lot of races (all sprints and quite a few mass start races) are decided. Top speed was one of the weak aspects of my skiing in previous years as no particuliar focus was directed towards sprinting. What really suprised me is how any improvement in sprinting is transferred to all other aspects of skiing. The improved balance and more efficient technique usually translates to better and faster skiing at reasonable paces. It is then possible to relax at slower paces and conserve even more energy. After a few sessions skiers should feel more confident on their skiis. If different grades are used then they will also develop their abilities to adapt their technique to the environment.