Step 1: Gather Materials
-string, just some plain white string found at your local drugstore or craft store, can also be yarn and some other strings I guess, although I havent tested those.
-a book you want to take a page out of
Step 2: Cut It
Step 3: Wet It
it must be pretty wet, although it doesnt have to be dripping (if it is, that may be too much)
Step 4: Do It!
pull it tight so it is straight on the page, and then pull it close to the binding of the book (see picture for reference - red = string)
close the book and hold it tightly closed for a few seconds (10-30, depending on how thick your pages are)
now open the book and examine the page - it should have a line where the string was where it is wet, and therefore very weak.
now just tear the page out carefully - it should tear cleanly on the line. if not, wet the string more and put it back on the already weak area and do it again.