Breadboard end joints are very difficult to make if your top is slighdy cupped. It pays to do whatever you can to ensure that the top is flat when you glue it up, such as clamping heavy battens across the ends. If your top is still cupped, despite your best efforts, you can clamp it flat to a workbench while you make the joints. You'll be flipping the top over now and then, though, which will be awkward.
You may also use a belt sander or a large plane, such as a No. 6 or a No. 7, to flatten a cupped top (Photo 1). This requires skill and practice, whichever tool you use. It's important to flatten both sides, because they will be reference surfaces for making the tenons. Measure the thickness of your top at both ends when you're done. It shovdd be the same all the way across, to make the joints easier to fit. It also helps if both ends are the same thickness.
Next, make the ends straight and square. The easiest way to do this is to use a router, a wide guide board, and a top-bearing flush trim bit (Photo 2, and Sources, page 73). Make the guide board from 1/4- or 1/2-in.-thick hardboard or MDF, about 12 in. wide and 3 to 4 in. longer than the width of your top. Clamp a sacrificial piece to the right side of the guide board to prevent tearout on the top's edge.
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