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. Then legions of Wraith Warriors are sent down to find any large number of humans hiding in the shadows often feeding on them as they go. To prevent the inhabitants from escaping the planet by Stargate, the Wraith would dial another Stargate to the planet. 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 down to as few remaining as can escape the wraith, then the planet will be bombarded with Wraith energy weapons from orbit, destroying all traces of the civilization and ensuring none of them survive. Some worlds have hidden their advancements from the Wraith in order to prevent this, most notably the Genii.
Part of the Wraith's standard procedure is to block a planet's Stargate in order to prevent escape, usually by dialing in from another planet, but when they tried to cull Earth, they simply brought a Pegasus Stargate into orbit which overrode Earth's Gate.
In 2009, the Wraith tried to cull Earth using the ZPM powered Hive Ship. During the event, Colonel Samantha Carter said that what they do is first a surface scan for military instilations, then target them with weapons fire from orbit, then the power grid to send the population into a panic and then they launch the Darts to cull the population. During this attempted culling, Earth launched everything they had at the hive ship (the only one that knew of Earth's location) including Atlantis, but failed and were nearly culled. Atlantis was nearly destroyed and with it gone, the Wraith would have been able to cull Earth unopposed, but the ZPM powered hive ship was destroyed from the inside at the last second by John Sheppard with a nuclear weapon he snuck in by flying an F-302 equiped with it in through the Dart Bay as it would have been useles on the outside. He and his team were able to use the hive ship's Stargate to escape to the Milky Way Alpha Site while the nuke destroyed the hive ship. Destroying the hive ship prevented the culling and the people of Earth were unaware of how close they came to facing the horror of a culling by the Wraith.