Using a straight edge, (4 foot level,etc.) clamp and take a blade off with a circular saw. A blade means the kerf width of the saws blade (1/16) if its 1/8 off take an 1/8 off instead, make a mark, lift your circular saws safety guard up (do thos with saw unplugged) line your mark so that your blade is on the outside edge is next to your mark (to the left of your mark) then make a mark on the right side of your saws base. This will be your straight edges mark,clamp,then rip away. You need to do this 1/8 even 1/16 too big is a big deal in cabinetry. Need more help or maybe some pics? Let me know.