As with what ToEleven said, don't try and cut with one pass, multiple ( careful) passes is best and helps stop the ripping, also a sharp blade is a must. They do get blunt fast so either sharpen with a knife sharpening stone/Steele or snap of a new blade if your using that type. I tend to resharpen my blade as you can easily go through a lot in one build. Another thing that can be useful is a set of French curves (ruler set) in metal if possibly as you can inadvertently cut into plastic ones, this just helps with cutting those rounded edges, but if freehand is working that's fine too.