I've done a backstab with everything from daggers to halberds. You have to be behind the enemy, and they can't be currently recoiling from a parry or block (if you wait a second though, you can backstab them just as they regain control.)
The best way to accomplish this is to sidestep or roll out of the way of their attack, and quickly maneuver behind them. Hit R1, and enjoy the damage. The "front stab" you talked about is a riposte, which needs to happen after parrying an opponent's attack - these are risky, since if you screw up you just let them hit you, but they do more damage than a backstab. Tap L2 before their blow connects, and then tap R1 quickly afterwards.
Daggers get a bonus to backstab/riposte damage, and the game classifies these as critical hits (in terms of Fatal upgrades and the like.)
u can do it with any blade wep. for the front on u have to have either a smaller shield or a short-medium blade to do a parry with. it take time to get use to the parry timing but once u get it down right after u parry jus atk n u will do the stab. for the back on jus get close to their backs and atk. make sure u dont have ur shield up tho i dont believe it works with the shield up well usually never does for me haha but its really all jus about timing
ya u have to hold R1 directly behind or almost behind them. i tried pressing really fast and it didnt come out. also dont block with your shield cause it wont come out. Ive done it with a DRAGON SMASHER!! so i think it can be done even with a shield?