The Dog's Harness
The harness in the photo is an X-back. Notice how much room the fore legs of the dog have. The transverse webbing allows for chest expansion.
The X-back wraps the dog's body and allows the dog to pull with the whole body. Harnesses can be ordered with padding on the neck and chest. Other options are reflective strips for night running and closed cell padding that is lighter and will not absorb water. Harnesses can be ordered online for 17 to 25 dollars depending on the options you choose.
You should not use a leash for a towline. A leash will be too short and will also be too jerky when a dog is pulling. The line needs to have a bungee section incorporated into the core of the line to smooth out the yo yo motion of the skier and rider. The bungee is a form of suspension that makes it comfortable for the dog and the driver.
The line should be at least nine feet long. You need the extra length beyond the length of a leash so that you have more reaction time in case your dog stops to poop or mark. Without the extra length, you might be in danger of running into your dog. If you have very fast dogs then you should consider an even longer line. The end of the line that attaches to the dog's harness should have a small brass swivel snap. The end of the line that attaches to your wheeled rig or skijor belt should have a loop. Here is a link that will help you make your own lines. For canicross, it is not necessary to have such a long line. You will still like a bungee section. A canicross line should be about seven feet long.
Always attach the line to the front and center of your rig. Do not attach the line to the grips of your bike. This is a common mistake that will make you fight for control of your bike with your dog. When the line is attached at the center at either the stem or the head tube, then your hands are free to use your brakes and control your steering. Do not use your skijor belt as an attachment when bikejoring or scootering. A line attached to your body will get you dragged in case of an accident. If the line is attached to the bike, then the bike may get scuffed and dragged for a short distance before the dogs will stop. Human bodies are much more expensive and painful to repair than a bike.
This is one of the places your dog team can take you. Left to right, Tempo, Flash
The Skijor / Canicross Belt