go to the framing shop and ask for scraps. Always put a board under the one you are cutting for a clean edge.
Use the scraps the practice your cuts. Since you are cutting from the back of the board you will need to over cut the back to match up the cuts on the front. Practice this. No matter how hard you try to clean up a mat that was noy cut right you will never be happy with it, at least I am not. Practice, practice and practice some more. heck, that is what scraps are for right?
Oh yeah I thought one or two people mentioned this but I will reiterate what they said in case you missed it. KEEP LOTS OF EXTRA BLADES AROUND AND CHANGE BLADE OFTEN