Rough trim the excess plastic from the specimen face with a razor blade, keeping all the tissue edges. Put the specimen chuck into the microtome arm, and put in a dry glass knife. Shave off enough plastic from the front (face) in very small increments (start with 1 μm slices, decrease to 0.5 μm, then 0.1 μm until you get a mirror smooth face). If you must keep all the sample, then start with a very thin shaving just to get the face smooth. You should be able to tell when you are into the tissue by looking at the thin shavings. Mark the top edge of the one that will become the top of your trapezoid-with a magic marker on the chuck.