Trim your beard precisely.
Step 1: Upper trim
Select a reference for the back part (i.e. ear against the metric scale in the tool) and for the frontal part (i.e. the tool against the middle of your lips) and trim your beard following the tool's curved part. Flip it horizontally and repeat
Step 2: Neck trim
Turn the tool upside down, align it vertically with the center of your neck and trim your neck beard. Flip it and do the same symmetrically. Finish by cutting the leftover bottom part
Step 3: Finish
Trim everything else between the boundaries provided by the beard shaping tool