Backstabbing in combat

[Abizmla]

So, I rolled a rogue, don't play solo very often but when I do I try and back stab whilst in combat.
This seems to be really inconsistent. Sometimes I can just strafe through the enemy and back stab as soon as I get past them and it will register as a backstab.
I was wondering if anyone with more experience as a rogue could give me any tips on this subject or any subject really. I haven't played this game very long.