Pull the trigger.

Open the side cover and scroll the drum to load it with cartridges through the open passage.

Close the side cover.

Scrolling the drum to expose the cartridge to the firing position.

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

When all the cartridges have been fired, cock the trigger, open the side cover.

Make a shot, then cock the trigger and put the next round in firing position.

When all cartridges have been fired, pull the trigger and use a spring-loaded ramrod to free the drum from spent cartridges.

Kindly look at the video of the Colt 1873 Peacemaker Revolver Scale 1:4 in action

Be sure before use to read the safety rules here.