I think that your best bet is to start her out lunging in a surcingle and side reins. However, proper adjustment is vital – I cannot stress the importance of this enough to you. Here's a guide to help you:
If you are unfamiliar with side reins, here is another sort of guide that you may find useful:
It's important to remember that since she's been off work so long, you need to start her back slowly. Also remember that she will be slightly unaccustomed to being ridden, and also the the tack/gear involved. First, lunging her with no gear (to make sure she knows what you're asking of her) is in order. Then maybe adding a bridle, but no side reins to the equation. Next, put on the surcingle and lunge her with that. When all of that is successful and both she and you are comfortable, put on the side reins and adjust them properly (see guide). Long at first, and then bringing them to normal adjustment.
When lunging, encourage the horse to reach and push from behind by swinging the lunge whip at her hindquarters. You want her pushing from behind with her back engaged, seeking contact with the side reins. This will really help her muscle, especially in developing her topline.
Take it slow at first, with short (but productive) lunging sessions, and then, as she develops and becomes more fit, the lunging sessions can become longer and more physically challenging.
When the lunging sessions have become productive, and you can clearly see improvement, maybe you can start acclimating her to the tack. When both she and you are comfortable, and you are ready, you can go for your first ride together! I strongly recommend doing this with your trainer. Don't rush the first ride – make it relaxing and pleasant for everyone involved! (:
When you are comfortable in an arena with her, take her out of the arena! Go for easy hacks and trail rides! These will be super beneficial for both of you.
Starting her back in to work, it's important to give her some days "off." If this means just handwalking her about, then that is what it means! As she gets more and more fit, and you are riding her, rotate days that you lunge her instead of ride her. Continuing lunge work with her is going to really help her (and you) develop. :)
Working with a trainer through this all is everyone's best bet(:
If you do not have have a surcingle and side reins, here are two products that are both affordable and quality:
While side reins are pretty useful and whatnot.. I wouldn't put her in them right away. I'd start with groundwork and sacking out, and when she's solid in that, start lunging her just w/t/c/halt/reverse, teaching her ONLY the cues so she responds right away. Sometimes it helps to teach them in hand whilst leading them. Meaning for the first few weeks, you say "walk on" when you want the horse to walk as YOU walk beside her leading her on the leadrope, then you start jogging and encourage her to trot on the leadrope next to you (NOT into your space) as you tell her "trot."
Since she hasn't been ridden in awhile, I'd focus mainly on walk and trot. Lunge her at a walk over poles, at the trot. Once she's got that down, then you can add the side reins in every so often so she gets reminded to use her back and seek the contact. After she's looking and moving lovely in the side reins, then start getting on her. Practice mounting up and getting off a lot making sure she stands still.
Then take her for a nice walk, with some trot if you feel SHE is ready for it.
Don't bring them back too fast or they may turn sour or give you problems. Give her all the time and "tools" she needs to be ready.
In hand training (you lead her around and w/t over things, through things, around things, on trails, etc.)
Lunge at the cues
Ride (while still reserving some days to put her in the side reins.)