What you need:
M8 screw (40mm recommended), M8 nut (for 13mm wrench), M3 screw, some random springs.

Put the M8 nut in to the slot that on the inner side of the stock, use the screw from the underside to go through the nut... This will keep your stock solid to the buffer tube

Run a drill through the M3 hole from the underside. I recommend a 2.5mm drill, the print is normally just not great for precision here, that's why I prefer to drill after... Put a smol spring on the inner side of the latch with a bit of glue. You can do glueless in theory but I highly recommend a bit of glue to keep the spring in place. Put the latch in place and screw it. You can also potentially drill a bigger hole and use an insert.

Lastly, "Pad-L" and "Pad-R" are exact mirrors, put springs in the holes with some glue to secure in place. Once you have them both on the springs push them together and through the little holes on the buttpad. These should lock in to the little holes on the back of the stock in order for you to not loose the pad and the battery. Button can be inserted in the center for extra protection and securing it better