Steps to Killing someone as spy:
1: Find target, preferably a Pyro since a Spy's hatred for a Pyro burns hotter than there flamethrowers
2: Disguise as a Medic or Scout and follow the target
3: When the target figures out that your not on his teams slash at his face hoping you'll get that lucky facestab before he kills you
REAL tips to killing someone as a spy:
1: Try and get behind big groups with slow moving classes like a bunch of Heavies defending a payload, only strike when they're distracted with your team.
2: Disguise as a class and act like the class, ex: Disguise as a Sniper and go near a sniper perch, disguise as a Engineer and stay back and pretend to patrol etc.
3: Stab your target before you reach them, this prevents the long backstab animation, it takes practice but you'll get the hang of it.
4: Avoid disguising as Medics and Scouts because you walk a lot slower than a Scout and you don't have your ubercharge meter as a Medic, making you easily detectable.
Have fun and keep practicing
