amp-jekyll | Build Accelerated Mobile Page versions of your Jekyll posts | Static Site Generator library

I am working on a jekyll project and already asked some questions on stackoverflow and jekyll talks site. This post is a sump up for the issue I have about setting up amp-jekyll. amp-Jekyll is a jekyll plugin. The full documentation is at this adress : https://github.com/juusaw/amp-jekyll

The tree of my project is the following

My Version of jekyll is jekyll-3.6.2 I installed the plugin using the Gemfile (gem "amp-jekyll", "1.0.2" # installed). I checked that the plugin is well installed. Then I followed the documentation:

1) I Placed the layout file (amp.html at https://github.com/juusaw/amp-jekyll/blob/master/amp.html) to the _layouts folder. I have a first question about that. I looked at this file and don't know how to use it. Have I just to copy it in the _layouts folder and that's all or have I to take a part of it and copy it to another file (and if so where ?). I can see in the body part the use of {{ header | amp_images }} or {{ footer | amp_images: false, 24, 24 }}. What does that mean ? . And what about the part included in head tag ?

2) Then, I Added amphtml-link to post heads (Add CSS styles to the html template). In my case I just copy the following code