During most of the period of this forum and for the most part, slings were for Military Muskets and Rifles and thus had sling swivels mounted for the slings.
The one main exception to that was civilian Jaeger Rifles, which had a sort of button headed screw that was screwed into the bottom of the stock behind the trigger guard and a swivel up front for the sling. That swivel up front had a screw that went through the fore arm of stock and through a special and extremely necessary barrel tennon lug that took the stress off the slender/somewhat fragile wood of the fore arm of the stock. (Military Guns had the same kind of barrel tennon lug for their front swivels as well and the rear swivel screw normally went though a strengthened boss in the front of the trigger guard.)
However, only an extremely small number of American style Flintlock Rifles were known to have had such swivels fitted. They certainly had the technology to do it, but it seems they just did not feel the need or wanted to go to the expense to do it. Civilian Percussion rifles and most smooth civilian guns also rarely, if ever, had swivels and slings fitted.
Some of us tend to think that the few times in the period civilians may have used a sling was something like when they took a deer and may have used a simple piece of rope to sling their rifle/smoothbore over their back to free their hands to drag the deer. Of course, they may just drug it by the antlers or head in one hand and held the rifle or smoothbore in the other.
A sling with loops on each end and perhaps an adjustment buckle somewhere in the middle of the sling would be all one would need for a sling that could easily be put on or taken off the rifle or gun. Personally I would sew the front loop shut and make it so the loop would be just large enough to slip over the front sight, barrel and fore arm. There are at least a couple ways to make the rear loop capable of going over the rear of the butt stock and tightened behind the trigger guard. One thing I would suggest especially for the rear loop is if you use any kind of metal buckle for it, make sure it won't contact the wood or it will scratch/ding/dent the wood.
Gus