Position the countertop piece onto the cabinetry for a rough fitting. You’ll want to verify that the piece is plumb with the wall (lines up cleanly against the wall) and level with the surface (is not sitting unevenly).
Mark any areas where the countertop must be cut for a joint using adhesive tape. Silestone joint pieces are available and can be cut to the exact size needed.
Wash down the cabinet top surface and the bottom of the silestone with a sponge and mild soap. There cannot be any dust on the surfaces or they will not stick together. Allow the surfaces to dry completely before continuing.
Cut the pieces to size using a wet saw, following the lines you made with the adhesive tape.
Rough lay the pieces again, ensuring that they fit plumb and are level.
Spray a thick bead of silicone caulking onto the cabinet surface.
Set the countertop onto the cabinet and press down firmly. Install the large pieces first.
Add more caulking to the sides of the cabinets where joints are being added. Press the joints into place against the cabinet, holding in place for several minutes.
Wipe away any access caulk with a putty knife.
Clean the top of the surface with mild soap to remove any remaining silicone residue.