I used to be like you heh, i started out with a super tanker character, but them i wannted to try the assassin, and lemme tell you, with that kind of damage you can kick some serious pimple infested, bloated orc ass hehe.
It's pretty simply, you hide in shadows, you know, either click the stealth bouton or just drink an invisibility potion, and move behind, and i mean really behind the enemy. Once you're in said possition, just attack him (But not before). That should give you a backstab. Normaly it sais "Backstab does Septuple damage" or something else, depending on your level.
Normaly, i'd advise seting your script to that of the backstabing theif. That way your character will always hide in shadows, and when he sees aponents, backstab them. Just remember to help him out, after backstabs, by drinking an invisibility potion, which should allow him to backstab again, either another enemy or the same stubborn one that didn't die, or isn't dead yet heh.
One other thing, I recomend giving him a pair of boots of speed. That way, when you're using just him to do clean hits, you can hide in shadows, backstab your opponent, and quickly run behind a wall or door, and hide in shadows again, then backstab again. ^_^ It's quite fun when you get used to it.
EDIT 1: You might wanna check this just in case
http://www.gamefaqs.com/computer/doswin/game/25804.html there's a cool Thief's guide in there, it tells you all you need to know, including weapons to use, how to backstab properly, and other useful stuff. Practicaly a "do-it-yourself" thief kit hehe.
