It currently supports diagnostics (i.e. finding errors and warnings), go-to-definition and hover, with limited support for autocompletion (PRs are welcome!).
As of PhpStorm 2020.3 support for psalm is supported and on by default, you can read more about that [here](https://www.jetbrains.com/help/phpstorm/using-psalm.html)
Alternatively, psalm works with `gtache/intellij-lsp` plugin ([Jetbrains-approved version](https://plugins.jetbrains.com/plugin/10209-lsp-support), [latest version](https://github.com/gtache/intellij-lsp/releases/tag/v1.6.0)).
Setup is done via a GUI.
When you install the plugin, you should see a "Language Server Protocol" section under the "Languages & Frameworks" tab.
In the "Server definitions" tab you should add a definition for Psalm:
- Select `Executable`
- Extension: `php`
- Path: `<path-to-php-binary>` e.g. `/usr/local/bin/php` or `C:\php\php.exe`
- Args: `vendor/bin/psalm-language-server` (on Windows use `vendor/vimeo/psalm/psalm-language-server`, or for a 'global' install '%APPDATA%' + `\Composer\vendor\vimeo\psalm\psalm-language-server`, where the '%APPDATA%' environment variable is probably something like `C:\Users\<homedir>\AppData\Roaming\`)
In the "Timeouts" tab you can adjust the initialization timeout. This is important if you have a large project. You should set the "Init" value to the number of milliseconds you allow Psalm to scan your entire project and your project's dependencies. For opening a couple of projects that use large PHP frameworks, on a high-end business laptop, try `240000` milliseconds for Init.