When backstabbing, circle a targeted opponent without blocking. You cannot backstab with the shield up.
When parrying, don't look at your parry animation, just press the button as soon as the enemies weapon starts approaching your face. Also, you have a bit of time after the parry to do a riposte, so you can step forward to get into a better position, switch a weapon, or two-hand a weapon for more riposte damage.
You can parry and backstab humanoid enemies who are either your size or about twice as tall. You cannot parry or backstab non-humanoid enemies or gigantic ones.