Run a thick bead of clear silicone adhesive on the top edge of the bottom cabinets. This helps form a tight seal between the countertop and the cabinets.
Place the first section of countertop on top of the cabinets, with the angled edge over the corner. Press the countertop firmly against the wall and down on top of the cabinet.
Apply clear silicone adhesive to the mitered, or angled, edge of the countertop.
Place the next section of countertop into position, pressing it firmly against the back wall, the top of the cabinets and the mitered edge of the adjoining countertop section. Wipe off any adhesive the seeps between the joints with a damp rag.
Drive 1 5/8-inch screws through the mounting blocks underneath the cabinets and through the underside of the countertop. Press down on the area above the mounting block while doing this so that you don't push the countertop away from the cabinets as you drive in the screws.
Cut two strips of 1/2-inch plywood to be used to join the countertop sections along the mitered joint. Cut the strips 2 inches wide and 6 inches long.
Place the first strip against the bottom of the countertop section, positioning it so that it spans the joint. Hold it a few inches away from the back of the cabinets. Drive two 1 1/4-inch screws through the plywood strip and into the bottom of the countertop, using one screw on each side of the joint.
Install the second plywood strip, using the same technique, but place this strip a few inches away from the front of the cabinet.