Once it's set up, the VanMoof app will automatically connect to your Electrified S2 or X2 whenever you're within Bluetooth range and the app is running in the foreground or background.
If your bike isn't responding, you may need to wake it up by plugging in the charger. If this doesn't wake your bike up, then try resetting it by pressing the reset button, which is located on the underside of the top tube towards the front of the S2 or the middle of the X2.
If your bike is awake but you're having difficulty connecting, try toggling Bluetooth off and on, and if that doesn't help then try restarting your phone. If you're on Android then make sure the location permission is granted and location services are enabled, and follow the guide at dontkillmyapp.com to ensure that battery optimisations are disabled for the VanMoof app.