Service Rifle Sling
You'll need a service rifle sling, either the web or a Turner or a Tam. The Tams are built like a mule harness. If you aren't draft horse class I'd go with a Turner. OK Webber usually stocks seconds at a deal. Just avoid any cheap imitations. Nothing at Walmart and generally nothing at Army Surplus stores.
Do NOT use Neets Foot oil or saddle soap on the leather. If anything, use Mink Oil paste, and don't use much. Leather is pretty sticky and hard to get in and out of for a while and a LITTLE paste works. The oils make them too stretchy.
Just like your match sling, the bottom section of a leather or lower clip on a web is unfastened from the lower sling swivel.
If leather, I'd unhook the frog on the lower section and then turn it back on itself and hook it in its own first or second hole so it won't pull through the swivel and hook you somewhere unhelpful while you are going from standing to sitting or prone.
The leather upper section is basically going to make a figure eight- two loops with the keepers in the middle. The frog fastening that section together should start out facing in toward the rifle. The hooks should be pointing down toward the buttstock. One keeper ought to be above the frog and one below. Your arm will go through the bottom loop of the "eight."
That's the start. Get someone to help you. It's hard to get a sling too tight around your arm or too high over the bicept. It ought to be VERY snug overall but you should be able to keep your wrist joint straight.
Sitting slings are usually a hole or two shorter than prone slings, so take a ballpoint and mark the holes. These points will shift as the sling stretches out.
Leather slings last a couple of years of hard shooting before they get too rubbery. Put it on a hunting rifle or give it to your nephew for a .22. I have a couple of old Turners in Iraq right now on M14s.
Web slings are rigged with the clip at the rear, the adjustable tab at the front and facing outside. Put your leg through one loop and your neck through another and yell until someone shows up to straighten things out.
Eventually, it will all be as familiar and supportive as a hug from an old girlfriend.