You will need to remove the upper receiver assembly as well as the buffer and buffer spring and extend the buttstock out.
Then remove the castle nut with your stock wrench. Sometimes this can be a bit of a chore. If the castle nut was installed with loctite, you will need to heat up the area underneath the castle nut to loosen the bond. I recommend a heat gun or a hair drier, either should work well. Be careful not to melt any plastic on the weapon. If you are melting anything, you are overdoing it.
If the castle nut is staked it may just need a little bit of elbow grease in order to loosen the nut. As the castle nut and end plate are removed, be careful that the rear takedown detent spring stays put. Now you will be able to twist off the buffer tube from the receiver. Be aware of the buffer retainer and buffer retainer spring as they will go flying if you simply just unscrew the buffer tube. With the buffer removed, you will simply replace the end plate on the buffer tube with your new sling mount CAR end plate.
Now you will only need to do everything you just did, but in reverse. When screwing in the buffer tube, be sure not to put it in too far. You want it to hold the buffer retainer (and spring) in place so that it cannot come out of place, no more.
Be sure the rear takedown detent spring is properly held into place by your new end plate. As you hold the new end plate in place, use your other hand to hand tighten the castle nut. You can use loctite when reinstalling the castle nut. Then use the stock wrench to tighten the castle nut all the way.
You may want to stake or restake the castle nut and end plate. Now you can reinstall the buffer and buffer spring and the upper assembly.