This is the wrong but here is a step by step:
If you have the old stock off and the buffer tube removed its easy.
Make sure you have that spring and detent and make sure you have lost the rear takedown spring and detent. Thats the first spring that should have poked out after you removed the stock.
Get your new buffer tube and thread it into the lower, just a bit and slowly. Make sure the nut on it is all the way back.
Now, put the buffer detent in the whole with its spring. Slowly screw the buffer tube in untill it reaches the detent and then push the detent down and turn the tube in till it holds the detent. Now look at the tube, there's a slot in the threads that has to be on the bottom. Turn it in till that slot lines up. It will still be loose, thats ok.
Put the other spring and detent in and slide the lock plate and tighten the castle nut by hand being careful not to squish the spring.
Then, tighten the castle nut with the tool. Just a bit more.
Insert the spring in the tube, the the buffer while holding down the detent.