How to get your dog to stay calm for harnessing and leashing?
It can be frustrating when trying to get your dog ready to go for walk and they are so excited that they are not sitting still for you. You have to give your dog something to do. I ask for sit and wait (or stay) for when I am getting dogs ready or getting their leash and harness on. If the dog gets up or acts wild at any point, I stop what I am doing and wait for them to calm down. I might ask them to sit and wait again.
Your dog wants to go outside, and if you allow him to act with high energy while getting ready, you are teaching your dog to act this way every time you put on the leash or the harness. Your dog will make a connection that if he behaves a certain way he will get to go for a walk.. You need that behavior to be something managable, like sitting nicely.
It is also important to get your dog to calm down for getting their harness on so that they are calm for going out the door. By helping him control his impulses now, you can set a tone for the rest of the walk.