Prerequisite: If you're coming from the legacy Pebble app (2016-era), you'll need to uninstall that first. You won't be able to pair or use any watches with both the old and new apps installed simultaneously!
To get started, head to repebble.com/app. Depending on your phone, select the appropriate link to your app store.
Install the Pebble app.
Open the app, then follow the onboarding process. Be sure to allow all permissions requested, or certain features on your watch won't function properly.
You're done! If you just received a shiny new Pebble watch, be sure to follow the guide here.