Steps 4 & 5: Cut curves
4) Cut curves with a jigsaw fitted with a wood blade. The blade will wear out fast; have a spare.
5) Along the top of the cabinet sides, screw on the scraps of wood flush with the cabinet top. Drape plastic over the cabinets and place the cement board on top. Then screw down through the board into the wood scraps using drywall screws. Attaching the board this way makes the counter easy to remove if you or a later owner wants to redecorate. Attach the foam plug in a similar way.