1) Local Lightning
Local Lightning works differently than the previous version of Local. This guide is for Lightning.
Install and activate the "Xdebug + PhpStorm" Local add-on. You'll need to restart Local app to complete installation.
Whenever a new site is added click its "Utilities" tab, then click "Add Run Configuration to PhpStorm" to insert "Local" with your site's URL as a "PHP Web Page > Server" in the Configurations, then specify its root location via Path Mappings.
2 min video walk-through (requires the following to be completed already... so continue below before trying to trigger an Xdebug breakpoint...)