Now take the soldering iron with a flat tip, plug it and switch it on. If you are using soldering iron for the first time, then read on its 'how to use' guide. Start with the primary step after the soldering iron is heated up. Hold the bumper in a way, so that its inner side is facing upwards. Now, you have to join the two parts of bumper, which are separated due to the crack by melting them and pulling and pressing them into the aperture.