If your thief is invisible (either by invisibility spell or by stealth) walk to as close to the creature as possible. Then, once you are right next to it, hit it. Your THAC0 is lowered by 4 while you are invisible, which gives you a good chance of hitting the creature. The damage you do depends on the level of your thief (levels 1-4, damage = x2, levels 5-8, damage = x3, levels 9-12, damage = x4 and levels 13+ = x5 damage). Note that only your very first attack counts as a backstab!
