Measure the width of your cabinet. Cut a 1-by-2-inch board to this length. Apply construction adhesive to the bottom of the board on the 2-inch side. Align this board with the front face of the cabinet and the ends. Nail with a finish nail gun every 8 inches along the length. Measure from the inside back edge of the board to the back of the cabinet where it touches the wall. Cut two pieces at this length. Glue and nail them aligned with the side of the cabinets.
Hold your crown molding against the cabinet so that the top edge slants away from the cabinet. Note the portion of the molding that is flat to the cabinet. Turn your miter saw to the left to the 45-degree mark. Position your crown molding against the fence with the portion that should be flat to the cabinet, flat to the fence. Cut the left side of the molding at a 45-degree angle while holding the board against the fence as described.
Tip the molding forward so that it is face side down on the saw. Measure from the narrow back edge using your cabinet width measurement. Mark the wood. Slide the molding to the left until you are at your mark. Pivot the miter saw to the right side at the 45-degree setting. Line up your mark with the blade. Tip the molding back so that it is face side toward you again. Press the board with the portion that is flat to the cabinet, flat to the fence. Make your second cut.
Bring your crown molding forward and tip it face front. Turn your waste board and see that the angle you cut created a 90-degree corner when used with the waste board. Measure from the cabinet or shorter side of the cut to the depth measurement of your cabinet.
Place your molding in the saw table face down. Slide the cut edge to the left. Rotate the miter setting to 90 degrees. Line up the mark with the blade. Tip up the molding and hold it against the fence the same as before. Make your cut. Tip the molding face down. Measure from the 90-degree cut edge using the depth of the cabinet measurement and mark. Slide the molding to the left. Pivot the saw to the left to the 45-degree setting. Line up the blade and the mark. Tip up the molding and cut.
Position the long molding across the face of the cabinet along the top edge. Test fit the ends. Apply carpenter's glue along the bottom back edge. Position the molding 1/16 inch below the seam between the 1-by-2-inch board and the top of the cabinet. Nail the molding to the 1-by-2-inch board using finish nails every 8 inches. Apply glue to the mitered end cuts on each side. Glue the bottom backside edge of the side pieces, and nail the moldings to the 1-by-2-inch boards. Keep the corners tight and nail one finish nail through the side and into the end. Be careful not to split the wood. Repeat for the second side.