Ideally, you don’t want to be practicing going up or down a hill when there is a big group of horses riding on ahead of you. That’s just setting yourself and your horse up for disappointment (and possibly disaster). Your horse will be too focused on staying with the herd to slow down and think about slowly placing his feet going up or down hill. Work on one problem at a time.
The best time to do this is when you’re by yourself, or if you’ve got another horse beside you. This is very difficult to teach a young horse or a nervous horse when you’re the last one in a trail riding group and everybody else has gone down the hill and they’ve taken off and your horse is the last one there. Of course he’s not going to want to get left behind, because when he gets left behind in the herd, he feels more vulnerable. Horses are naturally programmed to feel safety in numbers because if predators come, there’s no chance that one horse is going to be able to escape by himself.
Ideally, I like to train a horse on the trail by myself until the horse gets confident. Or, if I am riding with another horse, I make sure the other horse isn’t far in front of my horse. If you do ride with another horse, make sure it is a calm, older horse that can give your horse a little extra confidence.