First open Settings > Project Setting... and locate AppsFlyer under the Plugins section.

Setup parameters

Set your App_ID (iOS only), Dev_Key and enable AppsFlyer to detect installations, sessions (app opens) and updates.

This is the minimum requirement to start tracking your app installs and is already implemented in this plugin. You MUST modify this call and provide:
AppsFlyer Dev Key - Your application devKey provided by AppsFlyer.

Apple App ID - For iOS only. Your iTunes Application ID.

Is Debug - When enabled the AppsFlyer SDK debug logs will be printed (development only!)

Once your dev key and app_id is set you are ready to use AppsFlyer.

You will also need to make sure your package name is set up with a AppsFlyer Dashboard.