Oh boy, this could get REAL interesting. Ummm, you have the nylon strap. The rear of the nylon strap should ideally be on a swivel that is on the side of the buttstock. If your swivel is still on the bottom of the stock, you might want to configure a simple strap that runs through it and around the stock, and maybe from the strap over the butt and back. Reason for this is the bottom swivel will cause the gun to roll some. Ok, now you need to have some of the triglide swivels. For this it would be better if I tried to send you a picture.
http://www.arniesairsoft.co.uk/projects/3pt_sling/3pt_sling_howto.htm
You don't have to have the triglide #3 and you don't have to have the quick release buckle. #'s 1,5&6 you have to have.
Good luck. Another solution is if you have another rifle, buy a $25 Tac sling to get one to but side by side with it. Once you get the hang of how to set t up you could even make your own. They're no big deal.