Hi Jota, let me kindly disagree, what Andrew is referring to is to turn the single folder into a multi one, so you would not copy the drupal but explain to it to react on another site name. It is a bit like creating an apache virtual host, but on the drupal level.
For this, there is no change to the stack necessary, I believe you could even get around with no additional database, configuring prefixes for the tables.
What is necessary is to create a new site folder structure within the drupal installation. Now exactly this has changed on D8. I recommend looking up the instructions for this on drupal.org or somewhere "google'ish". Practically they should work nicely on the existing stack.
In the end, the module just install drupal. There is one catch though: The backup of the multisite might look slightly different from the usual one, as you might have decided to create a separate DB and might require changes to otherwise automatically setup config files.