Get the end with the sharp point and slide it behind and under the gills and out the mouth.
Then slide it through the ring and pull it till its snug. Tie the loose end to something strong like a rock or branch.
If you catch other keepers just slide the point under the gill and out the mouth again. Slide the fish down to the other fish and tie the stringer again.
Be careful not to damage their gills, cause that will usually result in a slow suffocating death. The reason you put fish in a stringer is to keep them alive and fresh, so damaging their gills defeats that point.