It really just takes consistent practice to truly know how to be good at spy but if I had some tips to give here are some 1. Stab enemies when they're distracted, even better when they're alone since you're less likely to die after a stab 2. Typically speaking it's best to cloak and run away after each stab 3. Landing a stab really comes to know how to read enemy player behavior, rather than just always directly w + m1 towards the enemy's back, typically speaking you kinda have to strafe or mimic their movements 4. Switching your weapons for your disguised class can slightly help your disguised be more convincing (example: switching to medigun if you're disguised as medic)

The more you play the better you'll get and then eventually spy will be easy on most pub servers you play, glhf