How to successfully pull this off?

-it seems random

-standing?

-crouching?

-how close?

THANKS!

hidden/behind is the best chance of getting a "backstab" And pretty close. Within striking distance but not so close the npc detects you in their "circle" which usually causes an npc to step away.

Sneaking power-attack with a dagger from behind is the easiest way.

If you mean the kill move where you literally impale an npc through the back, then you have to be standing and be behind the npc. In my experience, bashing or power bashing then quickly running around behind them and finishing them off will result in a back stab.

also, if you have a follower and the npc is agro-ing the follower it's really easy to just run behind them and get the kill move.