A proper musket sling will have a single buckle, and two folded, meaning doubled back, ends. At the ends there will be 8 holes. The ends double -back so the holes meet. You loop these ends over the sling swivels. You need two leather thongs. Now through the holes at each swivel goes one of the leather thongs. Each sling swivel is near two pair of these holes. You begin with second pair, not quite the closest pair to each swivel, and poke each end of the thong through the pair of holes closer to the buckle, pulling them through BUT you leave a small amount slack leaving a loop in the thong where it passes through those first two holes...then your two ends of the thong are on the other side of the sling. So, you poke the ends through the remaining two holes from that side, back to the beginning side, but crossing them as you go, so one goes / and the other goes \ and you end up with the thongs making an X on the side of the sling that is away from the stock. Now the thong ends are back on the side of the sling where you left the loop in the thong at the beginning. So you pass the ends of the thong through the loop, and tighten the loop as you take up any slack in the thongs. Here's is a photo where you can see the end of the sling attached to the rear swivel.
http://www.re-enactmentsupplies.co.uk/index.php?route=product/product&product_id=354 (click on the photo and it will enlarge and give you an idea what my cryptic instructions are trying to say
) When the sling is properly attached, and the musket is upright, resting on it's butt plate, the end of the sling passes through the belt buckle and continues toward the ground.
LD