I'm a volunteer for a not-for-profit project committed to conceiving a "way" to easy (considering the user audience not used to browse websites) and make attractive the registration of a new user on the home page Joomla website 3.9.
After a web search and a look in the Joomla section of the extensions on the related, I've just found that one of the possible options that meet my requirements is a free extension that enables the user to signup with a social account (Twitter, Facebook, etc.): SLogin.
I've found that the category under which I may look for a suitable extension is: Tutorial or website tour. But all of the ones I find on the web are not free.
If I was in the shoes of a user not familiar with registering a new account on a website, maybe I'd expect to find a guided procedure with popup windows or more than one webpage with plenty of information on any single step involved in the user creation.
I wonder if there is a template or extension (free) or any other option (not custom developed) that may do that job; I'm open to any suggestions based on the requirements above.
In addition to the options already mentioned, I've evaluated and finally selected a Joomla template with a slideshow extension on top that repeatedly displays the required number of slides to show how to register and/or login.
One of those slides will feature two buttons that will open a modal window to register or log in.