How a green screen works:
Firstly Green screening has many different names. Chromakeying and color keying are two different names. This is the process of singling out a particular color in an electronic image and then using computer software to make that color transparent. This allows another image, which can be just about anything you can imagine, to show through.
This sounds very simple and it is for the most part, but there are some things that can make in complicated.
Firstly is lighting the Screen. If the screen is not evenly lit this can cause issues because there will be different shades of green resulting in different variations of transparency. This can completely mess up a chromakey.
Make sure to light your screen properly!
A second issue is lighting you subject to match the image you are superimposing over the green screen. If the light, color, and shadows of the subject don't match, your audience will notice and t will effect your picture.
Make sure to light your subject according to your setting!
Finally edging. When using a green screen the color of the green can reflect on the edges of objects causing them to also become slightly transparent. This is fairly unavoidable but can be fixed in post with some work.