MEASURE TWICE, CUT ONCE is such an old carpenter’s motto that it now sounds trivial, but it’s the most sound piece of DIY advice I’ve ever been given. Far too often I’ve measured up, cut a piece of wood to the right length, and then found it doesn’t fit. Measure it, go to cut the wood, and then just before you cut, measure it again.
And if you’re making a bookcase, don’t make the mistake I made. Measure the height of the books, and then remember to add the thickness of the shelves before you fix battens to the wall and nail the shelves onto the battens. You’ll avoid a lot of jammed-in paperbacks that way.