Wonderboard is a cement based product that comes in 3 x 5 sheets and is available at home centers. It is usually used as a backerboard for tile floors and walls. Cutting wonderboard requires a few special techniques and tricks.
Measure and mark the board for cutting with your pencil.
Make the long cuts first. Use a circular saw with a cement board blade. Support the entire board before and during cutting to avoid breakage. The scoring knife can be used instead of a saw for a cut that goes across the whole board. Use a straight edge and press hard as you draw the knife along the cut line. Make several deep score marks and then snap the piece off.
Use the jig saw with a cement board blade to make small cutouts or other small cuts that are not straight. A drill with a masonry bit can be used to make a hole in the middle of the board to get you started with the jig saw.
Dry fit your cut board before installation. Put it in place and check that the edges line up before permanently fastening it.