How do you put a dog harness on a dog?

It would depend on what type of dog harness it is. If it is a fixed harness with two round circles connected by a single strap along the back, this would go on over the dog's head. More usual these days are step-in harnesses for dogs, whereby you would lift the dog's front paw and gently put the leg through one arm hole and reach behind the other leg and bring the second shoulder strap up, so the central chest area was covered by the middle chest strap and simply connect the parachute clips so that the harness is secure, not too tight, not too loose. Some dogs can be trained to step in the dog harness by simply laying it on the floor.