Git clone the WordPress develop repository somewhere on your hard drive:
git clone git@github.com:WordPress/wordpress-develop.git
Comment out the class_alias() functions in phpunit6-compat.php because these break PhpStorm code completion. (These files aren’t actually used by the testing framework, we only downloaded them so they could be included in the Project Configuration’s Include Path.)
PhpStorm -> Settings -> Languages & Frameworks -> PHP: Add wordpress-develop/tests/phpunit/includes to your Include Path.
Use WP-CLI to generate the tests scaffolding.
Write tests that extend WP_UnitTestCase. Look at the code in wordpress-develop/tests/phpunit/tests for examples.
