After you open the App, you'll need to create a Garmin account or sign in with your existing one.
Once you have signed in, in the Add a Device screen, select vivo->vivofit 2, and then add your personal details.
To start pairing, you need to hold down the main button of the vivofit 2 until it shows PAIR. Confirm that the code shown on the vivofit is the same as what's shown in the App to complete the pairing process. If there are other phones paired to the vivofit 2, make sure you turn off or their Bluetooth so they don't interfere in the pairing process.
Once paired, the App will start to sync with the vivofit 2. You can then adjust the Device Settings to your liking before starting to use the vivofit 2.
If you want to sync the fitness data collected by the Garmin vivofit 2 with the App, just hold down the main button of the vivofit 2 until you see SYNC. The App will automatically sync with the vivofit 2 and upload that data to your Garmin account in the cloud.