Got most of the setup from Boo's Scout guide, so here's something similar for
Spies... should be everything you need to know.
Basic Spy Tips:
1) You're a support class, so do some supporting: You're the only class that can
get into the enemy ranks/base and do some damage in secret, so use that to your
advantage, and make a mental list of everything the enemy has. Then use your
abilities to take out each one, in order of importance. If there's a Sniper
demolishing every teammate that comes near their team, find a good time to
backstab him. If a heavy weapons is just flattening everyone, do the same to
him. If he has a medic, take out the medic first so your team can do some
damage as well, or you may even get a double kill. After that, you'll want to
take out any sentries. Those make it near impossible for the rest of your team
to progress, so use your sapper to put them out of commission. After that, judge
your options and go after whatever's left... even the flag, if need be.
2) No fear: Don't be afraid of anything the enemy has. If there are a ton of
enemies in one place, you're probably not going to get away with any backstabs.
So be resourceful, pop one with your revolver, redisguise or cloak, wait for him
to come after you, then take him out. If an engineer is sabotaging all your
attempts to sap a building, try to catch him on his way to stop it, and backstab
him, or just shoot him. Chances are he'll defend himself before the sentry, just
make sure if you hurt him, the sentry's not in sight, because it can and will
rip you apart. Use all of your abilities to your advantage... combine them to
get the job done.
3) Timing: Make sure you time things correctly. You don't want to cloak to an
enemy base and have it wear off right when you're near the enemy forces. Find a
safe place, recharge, and head back in. Sometimes you can even find a room
behind the enemies, uncloak, and use your disguise to sneak up and backstab some
of them.
4) Communication: Make sure you communicate with your team well. Sapping
sentries won't help if your team doesn't know about it, and are too afraid to
come in. Backstabbing that Sniper is no good if people are still afraid to pop
their heads out, thinking he ducked behind a corner or something. Let them know
what you're doing and when they can proceed. You are a Spy, and Spies are
supposed to report the intelligence they've gained.
5) Take advantage of the "last disguise used" key: By default it's set to B,
it's good when on the move and you don't have time to pull out your cigar case.
It'll automatically use whatever disguise you used last [if it's a new game, I
think it'll pick a random one]. Just make sure you're not always using the same
disguise. The enemy team will catch on.
Movement:
A spy needs to know how to move well. Silently, quickly,
effectively.
Cloaking
Cloak [right click] is one of your most important movement tools,
know when and where to use it. It's obvious that you shouldn't cloak or decloak
right in front of the enemy, they'll probably shoot you anyway, and they'll be
at high alert for a Spy. Demomen and Pyros are the worst... grenades will
detonate as soon as they hit you, letting the enemy know where you are, and
dealing heavy damage. And Pyros should just be avoided at all costs, because
their flame, albeit short range, will stay on you, letting them AND the entire
team know exactly where you are. Anyway, know when to cloak. Don't waste it
while you're in your own base, and if you have a disguise on, you probably won't
need it once you're in the enemy's. The best time to use it is on the way to the
enemy base, even if you have a disguise. They're going to know something is
wrong when they have their o