A minimal working example of an OpenGL program that uses Dear imgui to implement a user menu. The sample uses GLEW as an extension loader, GLM for the mathematics and freeglut as window manager.
Note that the code is written in a way to prefer clarity in presentation over performance in the execution or good codding patterns.
All the sample code is in a single source file. However, the Dear imgui requires several additional files, those are contained in the imgui folder. Imgui files were taken from the library website and are not modified. Therefore you can always replace them with newer versions of the same files.
I tested the sample in an Ubuntu 18.04 standard installation. It also requires your graphics drivers already installed. And then, I installed all the libraries from the official repositories.
I provide a sample makefile that works on the environment described above. For other platforms or IDEs you can deduct what you need to do by reading it.