With the brace fully extended, loosen the Action Spring Guide Rod with with 1/2"socket and wrench.
Once loose, slide the guide rod out of the Receiver Extension, followed by the Aluminum Cap and Steel Washer, and Rubber Gasket. Make a mental not of their positions in the assembly.
Put the Rubber Gasket in the trash.
Remove the Brace Assembly from the Receiver Extension. This may require some force. Be mindful of the Rear Takedown Pin Detent & Spring when removing the Brace Assembly.
Slide your new Stock Assembly over the Receiver Extension. Align the anti-rotation feature of the Stock Base with the mating feature on the Lower Receiver. Make sure the Takedown Pin Detent Spring is compressed evenly in the process.
Insert the Recoil Guide Rod through the End Cap and Steel Washer, in that order. Slide this assembly into the rear of the Receiver Extension.
Before tightening, put Blue Loctite on the Guide Rod threads.
Place the plastic Recoil Guide Rod Alignment Tool into the Receiver Extension, over the Guide Rod. This keeps the Guide Rod aligned inside the tube while tightening. Misalignment WILL cause malfunctions.
Hand tighten the Guide Rod (righty tighty).
With the Alignment Tool in place, torque the Guide Rod to 20 FT-LBS using a 1/2" socket and torque wrench. Do not over tighten. This will squish the plastic Stock Base.