Hi, and welcome to the forum(:
First off, congratulations on your new lease!
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: Weaver 10-Ring Neoprene Lined Nylon Surcingle - Statelinetack.com
& Leather Elastic Side Reins - Statelinetack.com
But feel free to shop around! Some other good online stores are Dover Saddlery, Smart Pak Saddlery, and Cheshire Horse.
Good luck! And be sure to update us and let us know how it goes!