Angle the hair using a fine-toothed moustache comb. Make sure the moustache is dry, a wet one makes the hair appear longer and you end up cutting more
Trim first for shape. Start by cutting along the bottom and then the outer edges. Work from the middle towards one side, then begin on the other side.
Now, trim for length. Comb through moustache and cut hair on outside of comb. Be careful to cut slowly and carefully at first, You can always trim more later.
Once done trimming, comb through the moustache one last time. Clip any hair you have missed and use a few drops of serum to moisten it.