Here is a list of materials you will need in order to mount the sling:
Crosman R39 Airsoft Rifle
key ring with 1.125 inch radius
laptop shoulder strap with hooks
If the hooks on the strap are too wide, you will need another key ring with a .75 inch radius.
Step 1: Mount the Key Ring
Mount the bigger key ring on the two holes right above the barrel. If necessary, mount the smaller key ring onto the hole at the back of the stock.