I suppose its you i saw yesterday...

"Don't play spy" means you should play other classes to understand their logic. Then, act like the class you're disguised as so people fall for it.
Don't stare at enemies' backs; only look at their backs when close for a stab.
Prioritize Meds, Snipers, Engies, and overhealed Heavies/Soldiers. Remember, some players aren't deaf and will hear a backstab. It's better to kill an overhealed Heavy than a low-charge Meds (opposite if <70% uber).
Use voice chat to call out key info like "Medic has uber" or "Spy as Pyro" to be invaluable.

For more, watch Jhontohil2 or MrPaladin on YouTube for useful tuts. You can also watch other spies' gameplay (ignore the ultra-pro trickstab stuff) to see how they act in different situations.

Good luck g

+rep awesome player, unmatched reflexes, would love to team up again! 😉

synergize deez nutz in yo mouth

add me too, let's synergize in-game

command deez nutz into your mouth

commands the game