If you have the Assassin Training talent, you gain the ability to backstab or throatslit your opponents while standing behind them. To do this, you first need to know which direction your opponent is facing by typing ‘display $o’, or by adding ‘$o’ to your prompt.
An easy way to get behind your opponent is to move two feet opposite to the direction they are facing, then type either ‘rs’ (right slash) to slit their throat, or ‘rt’ (right thrust) to backstab them. If you have the Mighty Blow talent you can enhance your attack by adding a ‘+’ (i.e., ‘rs+’ or ‘rt+’).
Another option is to circle around your opponent with the ‘fl’ (feet left) and ‘fr’ (feet right) commands. You automatically stop once you’re behind them.
You can also check whether an opponent can be backstabed by typing ‘display $B’ or by setting your backstab colour (see ‘help colour’). Remember, in order to backstab or throatslit an opponent you must also have the weapon mastery talent for the weapon you’re using (see ‘help Assassin Training’).
Note that you can always perform a backstab on someone who can’t see you, even if you’re not technically behind them.