Now that I've found a good book press, how do I know I'm using it correctly? There are few circumstances a collector will find having a book press to be very useful. For instance, when a collector has purchased a "new" book, particularly if that book comes to him from an area with a very different climate, it can be good to allow the book to become acclimatized to its new home under light pressure in the press for a week or two. While it would be optimal for every library or book shop to maintain perfect temperature and humidity, that is unrealistic and so you'll see books adjust or change between seasonal changes. Sometime vellum will buckle with the slightest change in temperature and humidity, and while leather bindings need to breathe, they can also display signs of cockling, and these circumstances also would be remedied with a small book press. If you do leave your book in the press for a long period of time (longer than a week), make sure to take the book out occasionally to ensure additional damage isn't happening.
Applying pressure: Pay close attention to pressure! The book should be placed squarely centered in the press and the platen should be lowered until it presses firmly on the book, but you should not have to crank the wheel or handles. It is better to use lighter pressure over longer periods of time than heavier pressure for less time. If you can see the spine of the book buckling, you have applied too much pressure. If your particular book has a very wide spine, with shoulders that protrude above the level of the boards (cover), sandwich the book between two pieces of MDF with the edges of the MDF coming up to, but not over the protruding shoulders before placing the book sandwich in the press (see above photo for example).
(*If your book shows more damage than simple warping or cockling brought on by temperature and humidity changes, seek advise from a practicing rare book conservationist to discuss possible and proper repair and restoration.)