If your countertop turns a corner, it probably has two pieces, each with a 45-degree angle, or miter cut. These cuts meet to form the 90-degree corner. Chances are good that you have to do some scribing and trimming of the backsplash to get it to fit against the wall without leaving gaps and to make the front edges of the two pieces meet without any of the substrate showing. Follow these steps:
Place the countertop pieces in the corner to assess the fit.
Push the counters against the wall and into the corner as far as possible without misaligning them. At the same time, adjust them until the overhang at the cabinet face is exactly the same along the entire length of both tops.
Scribe and trim the back edges of each piece so that the rear mitered points fit exactly in the corner of the wall.
Typically, excess joint compound in the corners requires you to trim some material from this area. Additional trimming may be required to compensate for other wall irregularities.
Before you join the pieces, mark the ends where you need to cut them to length.
Cut the tops to length and apply glue to the laminate ends.
Apply yellow carpenter’s glue or liquid hide glue to both edges of the mitered joint and reposition the two sections. Either glue works well; however, carpenter’s glue sets up more quickly. If you use it, you need to work faster.
Connect the two pieces from the underside with the toggle bolts that came with the countertop.
Position the wings of the bolt into the slots on each piece. Tighten the bolt with an open-end wrench. As you do so, check the joint on the surface to make sure that the seam is perfectly flush. If it isn’t, adjust the countertop position and retighten the toggle bolts.
To ensure that the joint stays flush while it dries, place a C-clamp on the front edge joint and place pressure sticks on either side of the joint.
Pressure sticks are simply two equal lengths of board (usually 1 square inch in thickness) that are slightly longer than the distance between the countertop and the bottom of the wall cabinets. They need to be longer than that distance so you can wedge them in place to force pressure onto the countertop.