Measure the length and width of the island base with a tape measure and add an extra 1-1/2-inch overhang around the entire perimeter. Have the countertop built to those measurements.
Using 1-1/4-inch wood screws and a cordless screw gun, attach the table to the island using the cabinet braces in the island. The screw size may vary based on the depth of the countertop. Make sure the screw is no longer than the depth of the table so the screws don't pop through the top of the countertop. Repeat for all the braces in the cabinet.