It's a bloody good question!
It's likely you wont have everything sorted day one. If you have a rig thats comfortable in sitting, thats a massive head start, but don't worry if you haven't, find yourself an instructor/coach and they can advise. Make sure whoever you're working with knows you have back problems. BFTA Instructors and coaches are insured to do such, but cannot give medical advise, so if you feel pain/discomfort, stop and reassess.
I find that those with limited back flexibility, or just limited flexibility have a more comfortable position when the scope line and the part of the stock that is supported is wide. This does have some disadvantages, but those can be looked at later. But if you can shoot sitting in comfort, then the rest will come easier.*
Once you have that roughed in, you can then setup the rifle to it's optimum for sitting, move the butt, cheekpiece and lastly the scope into place. Once that's locked down, you can level the scope, zero it for windage, then do the ranges. Then it's a case of getting out and getting some experience. You won't know what you need to practice on until you get stuck in.
Targets are 8-55yds, so you'll need somewhere you can set that up. But I prefer to do as much work with an initial zero indoors first, as it's sooo easy to be off outdoors.
*yes i know top shooters may elect to build their stock around their weakest position like standing, but for a beginner, giving them the best chance to hit the most targets on a course (sitters) is where I suggest to start.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep