A:
According to most Ulama the method of shortening the moustache closer to the Sunnah is to trim it with a scissors or trimmer.
Mufti Shafiq Jakhura
Iftaa Department, Darul Ihsan Islamic Services Centre
I concur with the answer:
Mufti Zubair Bayat
Ameer, Darul Ihsan Islamic Services Centre