In a previous post from 2015, we described usage of Swift Mailer module to send HTML mail and mail with attachment. At this time, the module was not yet available for Drupal 8.
There is now a version alpha1 available. Let's go through installation process.
Because it has been rather tedious for us, we will try to explain the flow of the process as much as possible to help you save time.
The prerequisite is the installation of composer.
In our case we installed first on Windows inside a folder named F:\Program Files\composer2\.
The installation exe for Windows can be found here.