What I do is use a drill bit with the radius I want on each corner. Drill the holes out, and then use a dremmel on the slowest speed to connect the dots.
If you're not going to buy a plexi template and the right router bit, as the earlier poster suggested, and want to use a Dremel instead, find a piece of metal, such as a metal ruler, and use that as a guide. Put it right on the line you've drawn and lightly run the Dremel along it. I have found that there is no way in hell to make a truly straight line with a Dremel if you're free-handing it. The metal ruler as a guard works well; again, go light so you don't start to dig into the ruler or wear out your Dremel bit too fast.