I've built the following ship, which is capable of taking me to the Mun. However, with some careful piloting, I can make it into a stable Mun orbit with a bunch of fuel still left in the second-to-last stage. So if I want to land on the Mun, I have to decouple my lander module from the rest of the ship, and abandon any spare fuel I might have left in the tank.
So, I came up with the idea of using a docking clamp to attach the capsule to the main ship instead, so that I can leave my fuel in orbit when I want to land, and return to it later after returning to orbit from the surface.
Only problem is, I have no idea how docking clamps work. I'll learn how to approach docking maneuvers later, what I'd like to know NOW is how I should build the connection between the two sections to make this possible. I've circled the current decoupler in the image below.