Instructions
Use your mouse's scroll wheel until you reach the "Disguise tool." Choose a class in the menu and select it by pressing a numerical key on your keyboard to disguise yourself as that class. Use your scroll wheel again to select your knife.
Place yourself at the top of a set of stairs and wait for an incoming enemy.
Jump forward while an enemy is going up the stairs. This allows you to jump over the enemy's head.
Turn around quickly so that you are facing the enemy's back and left-click to backstab him.