First verify that rifle is empty, and then push out the pivot pin and takedown pin and remove lower receiver from upper receiver. Push buffer detent in and remove buffer and spring and set aside.

With the lower on an AR15 lower receiver vise block, loosen stock castle nut with a tele stock wrench.

Screw the castle nut all the way back and pull the end plate back exposing the takedown pin spring and pull the takedown spring out and set aside.

Hold your thumb over the buffer retainer and unscrew the buffer tube. Once it is backed off of the buffer retainer slowly release the buffer retainer but leave it in the lower.

Remove stock and remove the stock endplate and install the QD endplate so it looks like this.

Screw buffer tube back onto rifle all the way in until it comes up to the buffer retainer. Then press the buffer retainer back in and screw the stock around until it goes over the shoulder of the retainer enough to hold it in the rifle but not so far that the retainer binds against the tube.

Now put the takedown pin spring back in its hole in the receiver and carefully push the QD endplate up to the receiver. Compress the spring into the receiver without bending it over and screw the castle nut up to the back of the endplate.

Make sure the stock is straight and tighten the castle nut up against the endplate.

Press the button on the back of the QD swivel and insert it into the socket in the endplate.

Now you are ready to mount your single point sling to the swivel.

Here are links on the Windham Weaponry website to the various parts described in this article:
Q.D. Endplate
Q.D. Sling Swivel
Blackhawk 1 Point Storm Sling for AR15 / AR16