A book "Position Rifle Shooting"
But here is a dilemna. Despite your best efforts,you are almost certain to get some key things wrong.
Fundamental things. Which is to say you are about to build on a questionable foundation.
The real problem with that is developing bad habits and muscle memories that will take you in the wrong direction. These bad habits will establish a wall you will hit.
To get past the wall,you will have to unlearn the bad habits.That can be really hard.and at any time you may revert to bad habits.
So now you may be thinking "Gee,thanks. Way to rain on my parade. Nothing like a little discouragement!!"
No,I'm not going to leave you there. As Bart suggested,its best you find a coach.
I can't quite tell you how. It would be one on one,so COVID restriction should not be a problem.
How to search one out? You know your area and resources. I don't.
Ranges? Shooting Clubs? The VFW bulletin board? Your local gunsmith?
Are there any events you can attend to meet serious shooters?
You just might find an Old Guy who at least got serious with NRA smallbore.
This Old Guy might like nothing better than a reason to go to the range and pass on what he knows. You might breathe new life into him.
Or you might find an experienced service rifle competitor.
If you find someone,that book "Position Rifle Shooting" will help them coach.
I'll give you a clue as to whether you found a good one. If ,when you get started,he's looking down range at the target,you might want to keep looking.
He should be studying you. Position,sling,breathing,your eyes,your trigger finger.
Get a score book/log book. Make every shot a learning event.Write down all you can.
I watched an interview with Carlos Hathcock. He said the man who taught him his craft would take him to the range with one round, Carlos was to fire only one perfect shot. But he was to observe and record everything about that shot.Breeze in the grass,mirage,light,everything
I read of Ross Seifred. In his home South Africa,practrice ammo was a big deal. He was shooting a 1911. A practice session might be 10 rounds,or maybe two magazines. A few rounds,a mag change,a few more rounds.
His focus was to make those few shots perfect. It does you no good to practice poor technique.
Ross Seifred took that 1911 and won a world championship with it.
There is a CMP handbook that is well regarded.
The NRA used to have a course/qualification that could be done by mail if necessary. That might be a resource. It might help locate a coach.Shooting at 50 ft rimfire targets with 5.56 ought to work The 10 ring is the size of a 22 bullet. 11 bulls on a target.One for a sighter. Then one perfect shot at each of 10 bulls.
As far as setting up your AR,browse White Oak Armament. You don't have to spend a bunch of money there,but you can see how they set up target rifles.
Get Zelweigers book on the AR.
A spotting scope is a good tool.It lets you verify a call right then,while everything is fresh.
I'm probably right that,bang for the buck,a Rock River 2 stage National Match trigger is a good upgrade.Brownells (or others) can sell you a good 1907pattern sling. Browse Creedmoor Sports.
Good luck