It's all about the hit and run. Your thief is probably too weak to take on melee opponents directly, hence the backstabbing. So what you do is sneak up behind your target, while hiding in the shadows of course, and attack them with a melee weapon (a good magical short or long sword will do nicely, but use whatever you're proficient in). A level 5-8 thief has a 3x damage multiplier (lvl 9-12 4x, lvl 13+ 5x) and I think there is a THACO bonus for attacking while hidden, so it is really easy to do some quick damage with a backstab. The problem is you won't always finish off your opponent in one hit, or he may have friends with him. After you land your backstab blow, switch to your bow and take off running back to your main group (for tough opponents you'll want your party close by!). On the way, feel free to unlease a few arrows at whoever is following you.
Or you can implement the backstab in open combat. I always have my thief hidden in shadows constantly, which is important because you can't hide yourself when in view of an enemy. If a fight breaks out, I'll sometimes have my thief skirt around the edge of the fray, backstab one of the tougher or more dangerous opponents, and then retreat a few steps and fire away with bow and arrows. That gets in a good chunk of damage relatively quickly. With decent strength and a good sword a thief can do surprising amounts of damage with backstab
