LEGO base plate is a bit tricky to cut. It is too hard for scissor and too brittle for saw (and you will lose some spacing). Some use hot wire to melt and cut, but it is hard to navigate and, done incorrectly, will wrinkle the edge of the base plate.
One of the easiest way that I have used is using cutter and steel ruler. The whole process took less than 15 minutes.
Step 1: Create the Cutting Mark
Create a mark with sharp cutter on the place to cut. Use steel ruler as guide. This step might need to be repeated for a couple of time to ensure that the mark is deep enough.
Step 2: Bend and Flex
Once the cut is deep enough, carefully bend both part of base plate. Repeat step 1 if it is still too hard to bend both part of base plate
Step 3: Move Back and Forth
Move both part of the base plate back and forth until both part separate itself. You might need to pull both part apart once the cut gets to thin.
File each end of the cut with sandpaper, or cutter. Both cut of LEGO base plate is now ready to be used.