If you have taken down the magazine this far you can start screwing the tubes and caps. Either end of the tube will work.

The o-ring can be a little crushed and still work effectively just make sure the the o-ring is evenly in position.

The pliers technique used in the disassemble can also be used to screw the tubes back on to the rest of assemble.

The tubes and caps need to tightly screwed down but not overly forced.