You can use one of the inserts in the Blubel kit. Just wedge the corner of the insert in the clamp so you have extra padding around smaller handlebars. It should be stable and fairly tight.
Go to the Settings section on your Blubel app - then Manage Device. There you will be able to see whether you have a device paired. If you do not - press Add device and it should start the connection. If there are several devices around you, you will see them on a list.
Make sure that you’re not calibrating the device right next to your phone or where there may be some magnetic interference. Place the device in the mount on the handlebars. Turn the handlebars left and right like you would turn your handlebars during a ride, as indicated by the lights on the device.
You only need to calibrate your Blubel once. If you change position or angle of the mount on the handlebars or set it up on a new bike, please make sure you calibrate it again: go to Settings > Calibration.
You can make sure the app is no longer synced with your device by unpairing. Just go to Settings > Manage Device and press Unpair. Once you have done that, you can add another device, by selecting Add Device.