On the old stock:
Remove the buffer spring and buffer.
Remove the self-locking screw (the top one) from the end of the butt stock. This screw could require some soaking in oil/solvent first.
Remove the butt stock being careful not to loose the the pivot pin detent and spring (on the bottom right of the receiver).
Remove the buffer tube (you'll notice a nut like thing on the end of it to put a wrench on). Be careful and don't loose the buffer retaining pin and spring as you unscrew the tube. That's the pin and spring you will see just in front of the buffer tube.
Clean the threads on the receiver and apply a little anti-seize compound to them.
Install the new stock:
Install the lock nut and front retaining plate on the new buffer tube, in that order. Screw the nut all the way to the back end of the tube.
Screw the new buffer tube into the receiver, part way.
Insert the buffer retaining pin and spring.
While holding the retaining pin down screw the buffer tube in until it contacts the post on the pin. Then back it off a little until the pin in still held in the receiver by the tube but operates freely. Now make sure your tube is aligned properly.
Insert the takedown pin and spring in the receiver and hold the retaining plate against it.
Tighten the locking nut, by hand as tight as you can get it up against the retaining plate.
Now opinions will very on the next step. Here is what I do.
I back off the locking nut about 1/2 inch. Apply one drop of blue locktite on the tube threads and tighten the nut up as tight as I can get it by hand.
Insert the proper (new buffer and spring) they are different for the CAR stocks. Let the locktite dry and you are ready to go.