The picture that I put up is a 3yo and we started by ground leading in the hills.
I had a person with another horse and we led both horses up into the hills and just sat there until they relaxed.
This was done over a couple of weeks.
Next we mounted up and followed the same route.
When I go down the hills I go straight down and the steeper the better.
You have to work up to this and the horse gets use to doing it.
The horse learns to slowly take one step at a time and REALLY get their rear end under themselves.
At first I don't ask for the head or nose to be low and over time they just start to lower it because it is more comfortable for them on the incline.
If the horse gets a little excited then dismount and walk down the hill.
If you leave them alone they lower their head to see the ground also.
The same benefit is there for the climb.
Keep the horse at the walk and they have to really push up the hill.
Keep them straight.