So, you've purchased a Laser Designator!

The operation of one is simple, the first step is to ensure the Cairngorm has an intact, working gun. Once you've done this, you can just pull out the designator, hold shift and move if you want to do longer-range designation, and point at anywhere to designate a target, at which point the Cairngorm will fire the artillery weapon, and the designated area will shortly explode.