From your Server Control Panel you will be presented with a list of Operating Systems that can be automatically installed. If you do not see the Reinstall options open a support ticket and techs can enable that feature for you. Simply select the version of the OS you would like to install, below you will see options to add optional SSH Keys, select partitioning layout, specify root password and configure hostname. Once ready click 'Start Reinstallation' - the server will be rebooted and an automatic network install will begin. If you would like to watch the install process you can click 'Open Console' above. Once completed you will be able to connect to your fully deployed server.
If you have modified your servers boot order you may need to change priority in the BIOS back to Network Boot first then Hard Drive second. Alternately you can press F11 (when connected to the console using 'Open Console') during boot and select Network Boot from the boot menu. If you have any problems contact support.