Place the maple and walnut blocks in six rows of six to form a 12-by-12-inch square assembly. Alternate the blocks so that they form a checkerboard pattern. Place the end grain of each block facing up. You can identify the end grain by looking at the blocks. The end grain will have curved rings instead of straight grain lines.
Apply wood glue to the sides of the blocks, one at a time, where they adjoin the other blocks, and press the assembly together.
Center a frame clamp around the block assembly and tighten the clamp. Wipe off any excess glue that oozes out with a damp cloth. Allow the glue to dry at least 24 hours.
Remove the frame clamp from the butcher block assembly. Sand all surfaces of the assembly using an orbital sander equipped with a fine-grit sanding pad, until it is completely smooth.
Wipe off the dust left from sanding with a tack cloth just before you are ready to apply the finish. Rub mineral oil into the butcher block with a cloth rag. Allow the oil to penetrate the surface for five minutes and wipe off the excess oil with the cloth rag.