This is step 4 of our onboarding guide to becoming a reviewer
You can use any WordPress installation to test themes. So if you already have a test installation, you can go ahead and use that.
If you do not already have a test installation, The WordPress Core Team has excellent in depth guides on why we need a local install, and how to set one up.
Or if you have experience using Docker, we recommend WordPress-develop environment with Docker:
https://github.com/WordPress/wordpress-develop
If you feel like this may be too much to read, and you just want to get started, we also recommend:
Local testing environments:
Local by Flywheel (free). Local is available for Mac and PC. This setup is suitable if you are only working with WordPress projects, and if you prefer to have a graphic UI.
DesktopServer (free) DesktopServer is available for Mac and PC. This setup is suitable if you are only working with WordPress projects. Up to 3 WordPress installations.
VVV Varying Vagrant Vagrants (free) -An open source Vagrant configuration focused on WordPress development. VVV is ideal for developing themes and plugins as well as for contributing to WordPress core.