If you haven't done so already, I strongly recommend you follow the github guides to create your first sketch in the Processing IDE.
P5.js supports 2 modes, global and instance mode. In these guides I will stick with global mode because the code syntax is easier for p5.js beginners to learn. For those wanting to use instance mode in p5.js they can still use canvasGUI as it is compatible with both modes.
P5.js also supports 2D and 3D sketches and canvasGUI can display identical GUIs in both renderers without any code changes.
Using these guides
The guides (see index) are numbered in the order you should read them. As you progress through the guides the topics will require knowledge gained in the earlier guides.
As you become more experienced with using controlGUI then you might go straight to a topic of interest.
The control demos are very simple sketches that demonstrate a single control. You might copy and paste the code so that you can experiment configuring and using the control.