How to install a third party library for Printable module using composer (D8)?
This depends on how you have installed your Drupal site. There is a long description on how to use composer on Drupal.org handbook section.

The short version is this:


If you have installed Drupal 8 as you did with Drupal 7 using composer_manager module (which is depricated now)
If you have installed Drupal 8 using composer, you can require Drupal modules using composer, which automatically will download dependcies (assuming module developers has done their job by adding this information in their module's composer.json file)