Blood Kiss

The manner by which a Vampire turns a mortal into another Vampire is subject to much speculation. Known variously as the Blood Kiss, the Dark Awakening, Turning and the Red Ascension, this process is believed to involve the exchange of blood in some fashion. It was Queen Neferata's blood that gave rise to the first Vampires. The Blood Kiss is a highly secretive and personal affair, possibly unique to each Vampire, and the lords of undeath do not discuss it, not even with others of their kind.

Source

 * Warhammer Armies: Vampire Counts (8th Edition) -- pg. 6