You'll need more than just a collapsible stock.
You need carbine length buffer tube, unless the stock has one built-in.
You need a carbine-length buffer spring.
A-2 held in place with basically 2 screws at the end of the stock.
Carefully unscrew and remove stock watching for the little spring, takedown pin detent, on the back of the receiver that's held in place by the stock.
Don't let the spring fly across the room when removng the stock.
Next: remove buffer spring from buffer tube. Hold down buffer spring detent with thumb nail...work the spring out.
CAREFULLY unscrew buffer tube...capturing the buffer spring detent and spring before it shoots across the room.
While keeping the buffer spring detent compressed....screw on new carbine buffer, untill it comes into contact with buffer spring detent...locking it in place.
Slide Carbine Buttstock over the buffer tube, making sure to re-set the takedown detent spring without crushing it , and screw stock into place.
Replace the buffer spring...consider lubing it some with a hevay grease to quiet it..and you should be ready to go.
I've used a light loc-tite on the stock screws to keep it all in place, but many folks would never do it.
YMMV