Properly configuring the sling is one of the most important steps to accurate position shooting, but it's also one of the things that new shooters find the most daunting. Here's a great article with detailed instructions on how to use both the leather and web slings. Hopefully, it take some of the mystery out of the process!

Proper configuration of the Service Rifle Sling article

Another great reference if you want even more info is Glen Zediker's book Slings & Things. Although his highly informal writing style can be a bit unusual, he offers several ways to configure the leather sling and examines the pros and cons of each.