Snape drew his wand. His voice fell easily into the clipped, didactic tones he used in class. "The extraction of pensieve memories is a nonverbal incantation; a fact which I don't expect to pose a problem for a wizard of your calibre. Close your eyes, place your wand to your temple, and relive the memory you wish to share. As you do so, visualise your wand sucking the memory out of your skull, as the root of a tree will absorb water. If you are succeeding your memory will fade - until instead of a personal recollection it becomes indistinct, like a historical fact told you by someone else. If you are sensitive you may also feel a physical tug.
Wait until the sensations fade, then when you open your eyes you should see a wisp of memory clinging to the tip of your wand. It will respond to your intention to allow others to view it, and fall from your wandtip into the pensieve.
Similarly, when the viewing is over and it is time to retrieve the memory, insert your wandtip into the pensieve, and your memory will be called to your wand by your intention to recover it. Place your wand against your temple and attempt to relive the memory again. Your recall should sharpen as the memory is reabsorbed, until it is as vivid as ever."