Custom overlay Howto?

Hi good people. I've seen that on your demo page, at the very bottom, you have two buttons which open a custom overlay.

I found some related code in builder.js but not sure if this is what I am looking for. I need a custom button, which will allow to open sections only (besides sidebar) in an overlay. And if there is already some related code present, I would prefer to use that one, instead writing from scratch.

How is this best achieved? Take care :)