I like to do lots of hill work with my pony to get her fit, usually trot and canter :)
It depends on how long they've been off work etc. There might be a website or something to help develop a comprehensive 6 week plan?
But generally if they're very unfit you begin with just 20mins of walking for the first week.
Then add trotting in the 2nd week, then add in hill trotting, and build up to cantering and then jumping as their fitness increases. Generally there's no need to change their feed unless required or advised by a professional
Start with short ride but fill them with lots of exercise, them slowly increase the length of your rides and spread out the exercises through the rides. Keep the ponies food at a recommend size for the breed, age, hight and lessen hay/grazing time, do not lessen not feed unless recommend by a vet or trainer or experience person or if the recommend amount is less the what you give them already
With my pony I do lots of hill work short trots/canters up hills (made more intense as the pony gets more fit. Treks up and down hills is what I did to get my chubby pony fit, jumping also And putting him in a paddock with lots of pasture so he has room to run around when I don't have time to ride.