Pull the trigger.

Pull the ramrod, which serves as the axis of the drum, out of it (look at photo) and fold the drum out of the frame.

Load the cartridges into the drum, the side with the gray mixture should look towards the trigger.

Place the drum in its original position, holding the axle by the ramrod.

Pull the trigger.

Cock the trigger again.

Scroll the drum with your fingers to the next serif on the drum and you can fire again.

Kindly look at the video of the Colt Python Revolver Scale 1:4 in action