The edge of the receiver extension holds the buffer pin in place.
Back off the receiver extension castle nut, then while holding the buffer retain pin down, screw the receiver extension (buffy tube) until the edge of it touches the center pin, then back the tube off to index the butt stock.

Normally I would just tell you to finish the job off by tighten the castle nut, but if you have bought a cheap unit, you may have some fitting to do.

If when you back the tube off to index the butt stock, and the tube lip is no longer over the out edge of the buffer retain pin, you will need to mill/file down the bottom of the tube to index it correctly. If you had to file down the bottom of the tube, then chances you will need to do the same on the top.  Here, just use the end of receiver back loop (the threaded section), and scribe a line in the tube to see the amount that it will need to be shorted.
Note: if you leave the top long, you may get the upper to close ,but the back of the carrier is going to inside the tube (read inside the edge sticking past the receiver end loop), and getting the upper back open will require you to pull the receiver extension (al least threading it back a few winds).

Again, the receiver extension should be just a hair short of the center post, and fully over the out edge of the buffer retain pin. This is what holds the buffer retaining pin in place when the upper is shotgun open.