0. Have a setup with composer with a the web folder not in the root folder and install code sniffer with

composer require drupal/coder

1. Have a .githooks folder with executables

chmod -R 777 .githooks

2. Run

git config core.hooksPath .githooks

3. Make sure you have a pre-commit script .githooks/pre-commit

4. Make sure it is executable.

chmod +x .githooks/pre-commit

5. Voila. Next time you try to commit some code - code sniffer will check for Drupal Standards and best practices.