It's dead easy. You wait for zhe watchman to pass a dark alleyway and you rush past at zhe end of it. He gets startled and cautiously enters zhe alleyway, holds up his lantern, drops a classic 'who goes zhere?', only for zhe guttering light to illuminate a stray cat. he goes 'phew' but by zhat time you've snuck up behind him and are looming over him, claws and teezh at zhe ready.

Cue terrified scream, bish bash bosh, jobs a good'un