This is called a dummy cord. This is something we used in the military to keep our knives secure even if the snaps came undone. If you don’t have a leather hole punch, use a drill (backed by wood to keep from going through both strips of leather) or an awl can work, the key is to make it a snug fit for the leather that goes through.
Make four holes in a square formation through the front leather of your scabbard.
Loop the leather strap through the first two holes from the backside.
Insert the straps through the other two holes and slide the knife in behind. Tie knots where you want the straps to end and pull tight. The knot ends just dangle loose but will keep the strap from slipping back through the holes when you pull the knife and from being lost if the snap fails.