You can also just use the designer and look at what it created for you, study it and unravel how it works one call/object at a time. That would get you a long way. I don't know if the generated code is 'good quality' or not, though. Most of the time generated code is not as slick as human made. Its probably best to use this as a backup in case you want to do something and can't find good links / docs on it.
In the Qt designer, hit the text button. This will allow you to directly write in XML code, which is basically HTML for defining objects (though the rules are API specific). EDIT: oops, this was already recommended by jonnin.
I spent some time on it some years ago, and gave up. But I spent some time with the latest version this morning and got an example to run on Ubuntu.
For those interested:
- your include path must have imgui and imgui/backends
- you must build all the .cpp files in imgi
- you must select a .cpp file from imgui/backends that matches your platform. I used backends/imgui_impl_opengl3.cpp
- I also installed libglfw-dev
- link with glfw, GL and dl
Also note, the examples on the project page are incomplete, they don't set up the environment, create the main window, ... See some of the examples in imgui/examples for that.