Chances are you've seen this type of composition before. Well fortunately for you it's easy enough to do, so here's how to set it up.
The shot itself is incredibly easy to compose. First, you will need access to a decent sized reflective surface. It can be a mirror, a window, even water if it's still enough but for the purposes of this article, we'll just refer to our surface as a mirror. Have your subject pose relatively close by, if not up against, your mirror. You are essentially playing with how light bounces off a surface, so you will need to position relatively close by the mirror yourself in order to see the reflected version of your subject within the frame. Think about how a laser bounces off a mirror in a straight trajectory, well it's the same principle, so the closer you get to your mirror then the less you will be looking into it and the more you will be seeing your reflected subject.
As far as the mechanics go, that's pretty much it. It's a relatively simple composition but what makes it fun and new each time is your choice of surface, location, and lighting. Shooting with windows is one of my favorite ways to shoot because of how unidirectional your light is. It allows for a moodier, more dramatic look because you only have light coming from the one direction. I will often use a polarizing filter to play with the intensity of reflection for window shots. Sometimes it's fun to make the window more translucent and have the reflection become more ghostly.
The only real trick to shots like these is directing the eye line of your subjects. Simply tell them to look at the glass, to find your reflection in the glass, and to look at the camera that they are seeing in the reflection. That is all it takes to get some super fun, different, creative shots with your session. Below are a couple other examples of different variations of the exact same techniques. Try shooting both indoors and outside. Try using different surfaces and play with how you like the different opportunities with each. Just have some fun with it!