Many students have problems establishing the deuterium lock on a spectrometer manually. There is an excellent description of how to do so on a Varian spectrometer running VNMR here. The procedure however is always the same.
1. Turn the lock off.
2. Adjust the lock power and lock gain until you see a signal.
3. Adjust the field (Z0) until the signal is as flat as possible.
4. Adjust the lock power and lock gain to get a stable signal on scale.
5. Adjust the lock phase to maximize the lock signal.
6. Turn the lock on.