Your input knob (left) controls the input gain of the preamp itself.
The output knob (right) allows you to adjust the overall output of the preamp.
Ideally, you will set the output gain to 0dB and then adjust the preamp input gain by supplying input to the mic and inspecting the digital input metering on your DAW.
The value you should expect to see will depend on what sort of metering you are using, however if you are using a simple peak/rms digital meter try not to peak any higher than -6dBFS at the loudest point. Your RMS level should sit somewhere slightly north of -20dBFS. Ensure that there is no clipping by inspecting the DAW input metering and adjust to taste. Just make sure you leave enough headroom to capture the entire dynamic range of the signal you are planning to record.
Once you are happy with the gain structure, you can then use the output knob as a "fader" to adjust the overall output of the preamp. You can turn it all the way to the left to "fade it out" (full attenuation) and back to 0dB again for normal gain structure. You can increase the gain slightly should you wish to do so.
Note that the clipping indicator on this unit does not reflect digital clipping on the DAW input (ADC). The clipping indicator on this preamp indicates that the input signal is too high for the preamp to handle. Consequently, if you experience this, you should reduce the input gain slightly until the clipping indicator stops flashing.