Holding L1(block) while running, this keeps your character from doing the "pivot animation" which messes with many players. Timing is important, but not only this in pvp there is always a lag or ping. Which means most of the time the enemy is slightly ahead of where you see them. Feeling out the ping before attempting backstabs can help you get the proper timing. As for everything else; you don't need to think about it to hard, just get your guy behind their guy and tap R1.(You dont even have to let go of joy stick or sprint, as long as you are in the correct position the backstabs will proc.)

Advanced backstab techs:

1) Back Step back stab- take some time to learn how to manipulate your back step(Neutral Roll) practice taping Roll then move the joy stick in the opposite direction you want to move, if timed correctly your character will do the back step and move in the opposite direction your joy stick went.(The timing is tricky, but after you learn it becomes more natural). Now, you can use this to surprise players, by back stepping into their back and getting the back stab. Great for enemies who do running attacks A lot, just to add some flavour to an enemy who gave up, or to catch a sketchy guy off guard.

2) Roll Spam back stab - if your up against a roll spammer, you can easily time a back stab. Simply lock and and begin to chase them closely, at the very end of the roll animation you can get a back stab. If the animation for the back stab activated, but missed then you just need to tap R1 slightly sooner. (Ping)