Start slow. Keep it fun. Kick your skis, not the dog.
You must enjoy cross country skiing. If you don't, you'll never convince the dog that this is a good idea.
Set aside the notion that the skier is being towed. The skier, like a good jockey, does everything possible to unburden the dog, allowing it to either reach its maximum speed or maintain a slower pace for distance. Stopping competently on skis is a must. Not being able to stop endangers the dog and yourself.
All dogs pull instinctively; but certain breeds possess the drive, conformation and strength to excel in sprint races while others displaying those same positive traits, plus thicker coats, wider paws and a tougher psychology are better adapted for distance races or touring. An average of 55 pounds is a good rule of thumb, but bigger in this case truly is better.
You'll need a properly fitted harness for the dog, a tether line and a bungee section, a waist belt or climbing harness for yourself and a pack to carry gear and pooper-scooper bags (small trash bags or plastic newspaper wrappers work well.) Finally, be sure to brush up on trail protocol and go out and have some fun!