Identify the type of bus you need to set up. For example, create a stereo input bus to route stereo instruments, and a mono input bus to route single mono instruments.
Name your audio hardware connections. Click the "Devices" menu, choose "Device Setup" and select your audio card from the list shown. Click on a port name in the "Show as" column and enter a new name. Do this for each port shown.
Click on the "Devices" menu and open the "VST Connections" window. Use this window whenever you need to add, remove or modify a bus.
Select either the "Inputs" or "Outputs" tab. Pick the former to add an input bus and the latter to add an output bus.
Click the "Add Bus" button. Select the "Mono" or "Stereo" option to create a bus of either type. Assign a port to your bus by clicking the "Device Port" column.
Route any audio tracks you want to play back to your newly created output bus. If you've created an input bus, select the device port that will be feeding it sound.
Toggle the appearance of your buses by using the Nuendo common panel. The "Hide Input Channels" command excludes input buses from the mixer window. The "Hide Output Channels" command excludes output buses from the mixer window.