Im looking for a step by step explanation.
Alberto,
I was looking for that as well. However, after a little digging and reading, I realized that a PWA app is just a disconnected Web app with some other things added to spice it up and make it behave as a PWA. Although I have not gone down the path yet, I would follow the Disconnected Web App and then tweak it to become a PWA. That is what I have found.
Ron Jolda