Loosen the screw.
Pull the trigger and pull out the axle and drum.
Load the drum with cartridges.

Install the drum with the axle in reverse order on the gun.
Screw in the screw.
Set the drum to the firing position (the cartridge pin will look straight up).
Fire a shot.
Scroll the drum with the cocked trigger.

Kindly look at the video of the North American Arms Revolver Scale 1:6 in action