Slip the harness over your dog's head and adjust both straps to have a comfortable fit. If you can put two fingers between your dog's body and both the neck and chest strap, then thats a good start!
It is important to check the space between the straps and the armpits. We recommend having at least 2 fingers width of space to avoid rubbing.
Once adjusted, the harness should easily slip over your dogs head with no force.