Place the memory foam topper on the mattress or frame to measure the size you wish to cut. Put one corner in place so that you only have to cut two sides.
Measure the width that you wish to cut and mark the topper with the marker with a few small lines every few inches.
Measure the length that you wish to cut and mark the topper with the marker in the same way as the previous step.
Connect the lines with the marker to make a solid line. This will make it easier to cut.
Move the topper away from the mattress and bedroom before cutting. This will keep you from damaging the mattress, and the cuttings won't make a mess on the bedroom floor.
Using the electric carving knife, slowly cut down one side, following the line Move the knife slowly and don't force it through the material.
Cut the second side by following the second line you marked on the topper.