Vampires are created through a process called the Embrace. The Embrace is similar to normal vampiric feeding - the vampire drains her chosen prey of blood. However, upon complete exsanguination, the vampire returns a bit of her own immortal blood to the drained mortal. Only a tiny bit - a drop or two - is necessary to turn the mortal into an undead. This process can even be performed on a dead human, provided the body is still warm.
Once the blood is returned, the mortal "awakens" and begins drinking of his own accord. But, though animate, the mortal is still dead; his heart does not beat, nor does he breathe. Over the next week or two, the mortal's body undergoes a series of subtle transformations; he learns to use the Blood in his body, and he is taught the special powers of his clan. He is now a vampire.
Some vampire clans Embrace more casually than others, but the Embrace is almost never given lightly. After all, any new vampire is a potential competitor for food and power. A potential childe is often stalked for weeks or even years by a watchful sire, who greedily evaluates whether the mortal would indeed make a good addition to the clan and the line.