Their strategy in a culling includes the arrival of Wraith Hive Ships over the planets. The Wraith would later send darts to the surface and "scoop" up anybody they can find until only a handful of people are left. To prevent the inhabitants from escaping the planet by Stargate, the Wraith would dial another Stargate to the planet, to prevent them escaping. John Sheppard and Teyla Emmagan witnessed a culling from a cloaked Jumper, where there was nothing they could do except watch. (SGA: "Letters from Pegasus")
The wraith purposefully leave some of the planets population alive so they can repopulate in time for the next culling.
If a planets technology becomes advanced enough to potentially threaten the wraith’s dominance over the galaxy then that planet will immediately be culled, however some have hidden their advancements from the wraith in order to prevent this, most notably the Genii.