They're pretty easy to assemble. The transmission slides over the refill and screws into the coupler that is pressed in to the lower barrel. Be sure to screw it in tight so it doesn't unscrew when the pen is engaged. But don't lock it because that's how the refill is changed. Then the press the cap, clip and cap holder together, then press the whole assembly on the barrel.
You can print the instructions from the PSI website if you don't have them.