Open the folder with the project, run ./gradlew buildPlugin (for Unix-like) or .\gradlew.bat buildPlugin (for Windows) to build the plugin (or run "Package plugin" configuration)
The built plug-in will be at the build/distributions in .zip format, install it with Settings -> Plugins -> Install plugin from disk
Reload your IDE
How to run (developer)
Open the folder with the project, run "Run plugin" configuration, wait for the other instance of IDE to run