This should get you started: HorseHelp: Building up your horses back, In-hand and Under Saddle
Keep sessions short, start with ground work. Then move to lunge work (minimal) and then start riding her (you can do a mix mash of all options, but keep the riding sessions short and easy to start with.)
Think about it this way. Imagine you are going to take part of a triathlon. You wouldn't succeed without conditioning and training. Now you don't just start out biking 10 miles or whatnot one day and then running 15 miles the next day, etc. You start off maybe walking 30 minutes, and then build up to an hour over time, then maybe go to the gym and do some leg presses or swim or something. Over time you build up to do a triathlon (and I'm not knowledgeable in this area so forgive me!)
So now you have a horse. They need muscle in order to become better and more balanced and whatnot.
Start small, work on belly lifts and ground stretches/yielding. Then move onto walking. Walking over poles, in circles, serpentines, on trails (start a short distance, and increase) up and down hills. Then move to trotting. Trotting over poles, circles, serpentines, up hills, down hills (carefully.) Then work in a few sessions of cantering. I'd start asking for that canter on the ground without weight on the back so the horse can figure out its balance. Then over time, add in more cantering. Then once the horse is balanced on the ground, hop on and walk, trot, canter.
Shorter more frequent sessions are better than long sessions.