I actually hung the harness in the stall, LOL. My little brumby was wild enough that he needed that. Terrified of blankets('The Cloak of Death and Despair'), never saddled, barely halter broke, unhandled for years...
I think the most difficult piece of harness for them to get used to is the breeching. I left that off for a while. The closed bridle was no problem.
For first time harnessing, I had already spent a good long time, draping lead shanks, rags and such over his back, working up to a little bit of lightly letting the lead shank slap his legs or back or rump. Lots and lots and lots of that. If your horse has been trained to back away from spinning or twirling lead ropes, here's where that becomes a problem, LOL.
Once we got to where we could put on the breeching, I put the breast collar on, tied up the traces, and led my pony - a LOT. Lots and lots of walking. For a good long time.
Then started longeing, with only the surcingle and a halter at first, gradually add in each piece of harness.
I think one thing a lot of people do is not let them canter in harness. I understand that logic, but what happens if the animal does start cantering, and all the components of the harness start moving differently and sounding different?
So my little guy also cantered on the longe line, with each piece of harness gradually added. And given his first reaction, I am very, very glad it didn't happen some day out on a driving trail while hitched!
After a long time of longeing at all 3 gaits with all the harness, I started ground driving.
I did not want to hitch the traces to anything and have a panic, so I held the traces behind one elbow so I could exert some traction on them, and the lines in my hands (I only know how to drive Hungarian style or two handed).
I first started longeing with the traces dangling down and two longe lines, so he was getting used to lines laying on his body while he was moving. Then I switched to walking to one side or behind my pony, and putting traction on the breast strap so he would feel how it feels to need to pull into the breast strap.
Then I had my little 'fake shafts' that we hitched up and he went with those.
Then we hitched.
But we did quite a bit more before we felt ready for that. The cart was parked across his door and his hay was put on the cart. You want to eat, you get used to the cart. So never have we had any fear of the cart in general.
We spent a good long time with my SO pulling the cart(yes he got a medal) and me walking the pony, first behind the cart, then on each side, then in front of it.
THEN we hitched. The pony was still rather startled when the shafts touched his side in a more fixed way. It just does not feel the same way the 'fake shafts' (training shafts) do. They can feel that there's weight to the cart and that the shafts are not just laying there.
So we spent a good long time, hitching, unhitching, coupled with lots of treats, chit chat and rewards.
So now we are up to today. And this spring, we'll start up again, as long as my health continues to hold. Hopefully will be driving outside within 2 months of spring, but if it takes my little wild man a bit longer I just do not care.
My goal is a pleasure drive at the local park, and some day, a pleasure driving outing with the driving club.
But I am in no hurry and whatever we can accomplish is fine. Pony is still our little world champion, and always will be.