How do i sneak up behind someone and kill them?

This is just a guess, but I reckon that if you sneak up behind them and stab them they will die.
Therefore you have then sneaked up on someone and killed them.Alternatively you could RTFM

that depends on your status
if it is ateplar chnces thy wiil see you before you can kill them
if they are anyone else just go behind them chose hidden blade and when it shows assasinate press it