There are some good positive horse gentle horse trainer out there. Books and video that can help you too^-^
When you like to learn from good trainer...this one i did watch many video from, he seems to have a gentle hand and calm way to train horses, maybe look into barryhook2 on youtube (w w w.horsedrawnpromotions.c o m
,in the uk, He trains horses to be safe, confident and happy in any sphere of harness work...with right harness and bite for every horse.
Also some good trainer in netherlands with friesian and other horses....or Matthes Röckener
Or endurance driving , or classical dressage driving....
....
Start with good ground work, double lunge /long rein driving in the arena and outside on street and backroads and forest streets...a helper is always nice when possible^^, good desensitization and anti-startle training, like pulling bag with loud things inside in front of it, then next to the pony, behind it...Training with part and full harness on, walking around the marathon carriage/gig/ carriage someone pulling the carriage behind you and the Pony , pulling a tyre and so on till you can put the harness and carriage on the horse, and put ot of, to walk some steps, to being lead , to driving with someone next or in front or behind on bike or calm horse...best is an longer driven very calm teacher horse, together for first times in front of the carriage.
Not sure how well trained you're old horse still is and how calm, but maybe it can show youre younger some things, like being in harness, walking with it, walking together in front of you( helper is good for those parts too) and so on...
looking for right harness, sheepskin harness pads or something like this, quick release hardware, a good carriage/cart/ gig with good modern tyres, good brakes, protective equipment for driver(helmet, gloves, body protector )
But look into professional training or help.....or work with books and good tutorial video....and go as fast and safe as good for your Pony and you....train even under harness and in carriage with desensitization and anti-startle training, there are always new things and one has to deepen what you already know, to not forget it.
Take everything i say with some care ...i only did drive fully trained horses in a work tryout for a barn....
... and....i only started some basic back then but never gone more than this with own pony,as i could not find help and a trainer to go on, and now Do not have a horse anymore.
Maybe also look into...for training help? Or other that are near you

Like.....Britishcarriagedriving.co.uk/introduction/have_a_go.htm
britishcarriagedriving.co.uk/index_m.htm