How do I add an overlay on my responsive web page?

We are building a product, and we want to demonstrate a new user that how to flow with the system. I've seen some websites and apps showing a dull overlay on background and putting instructions over the screen.

Example:

For First time user: i.imgur.com/N92LD7n.png

For Regular user: i.imgur.com/XMX9fWW.png

I want the similar thing. I hope, I am clear enough. :)