Instructions
Specialize in one-handed weapons. Backstab is used when you only have one weapon equipped, so you can up your damage by increasing your stats with the weapon. When you level up, you will have the opportunity to increase your stats and abilities.
Increase your character's strength. A large part of the damage for Backstab is determined by your character's strength. Upping your strength attribute when you level can significantly increase the damage you output with the move.
Equip a dagger as your one-handed weapon. Swords and axes are efficient at performing successful backstabs, but daggers are designed around the move. They usually don't output as much damage as a larger weapon, such as a sword or an ax, but they have a much higher chance of performing a critical strike.