Start lunging under the saddle and try long reining her ! Then hop up and take it easy for the first few weeks
I would start gradually on a lunge line with the horse tacked. Start with 5 minutes of trotting and lengthen time from there to build stamina. If horse is sweating and winded you need to back off as you are moving too fast. Using a lunging system or side reins will help to build muscle that a horse who has been sitting, lacks. Once the horse has muscle built back up and some stamina you could put a rider on. This can take months depending on how long the horse has been sitting.
Little red cowgirl on YouTube did a very good series on this. It’s about breaking her horse chobbit but it’s the same principles as what you should be applying to your horse. Check it out
How long do you call a long time? And with are horses if it’s really long we do the go through the process of backing a horse again also you can lunge your horse with tack on then add weight to it to get the horse used to having tack and a person on their back!
Hope this helps