Blueboard is a specialized type of drywall that’s readily identified by the blue coating on one side. It comes in sheets which are the same size as drywall. Because of these similarities, blueboard can be sized and cut in much the same way as drywall.
Step 1 - Measuring
Using a tape measure and a pencil, mark the length of blueboard that you want on both edges of the board. Connect the points using a metal square for a proper 90 degree angle.
Step 2 - Scoring
Using the metal square as an edge, begin to score along the line you’ve made with a utility knife. Bear in mind that blueboard is thicker than regular drywall. It contains several layers of gypsum. After scoring deeply, you can try to break the board by putting a piece of 2 inch by 4 inch lumber under the cut and pushing down.
Step 3 - Cutting
The scoring probably won’t be enough to split the board. To complete the cut, you’ll need to use a circular saw and run it along the scored line. This will give you a much cleaner finish to the cut. Smooth the edges with sandpaper.