This is according to http://docs.drush.org
1) Install a site-local Drush and Drush Launcher It is recommended that Drupal 8 sites be built using Composer, with Drush listed as a dependency. That project already includes Drush in its composer.json. If your Composer project doesn't yet depend on Drush, run composer require drush/drush to add it.
2) To be able to call drush from anywhere, install the Drush Launcher. That is a small program which listens on your $PATH and hands control to a site-local Drush that is in the /vendor directory of your Composer project. If you skip this step,
3) Run Drush from Drupal root via ../vendor/bin/drush. In that case Drush's bash integration and custom prompt won't work. Run drush init. This edits ~/.bashrc so that Drush's custom prompt and bash integration are active.
4) To use a non-default PHP, edit ~/.bashrc so that the desired PHP is in front of your $PATH. If that is not desireable, you can change your PATH for just one request: PATH=/path/to/php:$PATH drush status ...`