My fixed telestock has a flat portion on the end, and it sits over the retainer...you can't really go past where it's supposed to be, it'll just hit the retainer, then you can't screw it in anymore...there's not another full turn left after it makes contact with the retainer.
It should be screwed in far enough to catch and hold the retainer...then you're done. Put the takedown pin in, then the detent and spring. You need to push that little plate down so that it pushes the spring and detent into the receiver...then se a CAR stock wrench, spanner wrench, etc, to tighten it...I'm sure there's a preferred amount of torque, but I just did mine 'til it felt nice and tight.
That little plate can be tricky...getting the spring to smush down correctly into the hole in the back of the receiver.
I usually do the front take down first on rettaching upper to lower...it doesn't matter. Usually works better with hammer down, but not an absolute necessity. Don't even worry about the charging handle.
Resist ALL temptation to pull the hammer on the lower when it's not attached to the upper, and with the bolt/carrier installed...it could crack your receiver!!