Psalm Requires PHP >= 7.1 and Composer.
composer require --dev vimeo/psalm
Psalm will scan your project and figure out an appropriate error level for your codebase.
Then run Psalm:
Psalm will probably find a number of issues - find out how to deal with them in Dealing with code issues.
Using the Phar
Sometimes your project can conflict with one or more of Psalm’s dependencies.
In that case you may find the Phar (a self-contained PHP executable) useful.
composer require --dev psalm/phar to install it.