This is a great step by step guide on how to make 3D game shaders in OpenGL: https://github.com/lettier/3d-game-shaders-for-beginners
The guide goes through how the shaders work with text based explanations and many times there are also images showing what is going on. It also seems to be actively worked on, as several new shaders have been added since I last looked at it a week ago.
