Just download the latest .so file for your architecture from the package registry or build it yourself with make build (requires Go and Docker). This uses Gotify's build tools to build against the latest version. The .so files are compiled to build/gotify-postal-webhooks*.so.
Then simply move the .so file to the Gotify plugin directory and restart Gotify.
Activate the Plugin, then go to the plugin's details panel to retrieve the Webhook URL. You can also see how to configure your Postal instance details there. If configured, clicking messages redirects you to the Postal message dashboard.
The parsed payload is sent to the automatically created "Postal Webhooks" application channel along with all neccesairy information. The channel can be renamed.
All Webhooks for Postal v3 as documented here are fully implemented.