After the brain has been fixed, you will want to "block" the brain before sectioning. Blocking the brain only takes a few seconds, but if done properly, it will greatly facilitate the remainder of the histological process. If you are sectioning to verify electrode placements, it will take fewer sections and the electrode track will be clearly visible. To block the brain, you use a razor blade to trim away excess tissue. You want to trim the block so that the anterior and posterior faces are cut at angle where sections will be taken in the orientation of the stereotaxic atlas. If the brain is properly blocked, your sections will closely resemble the images in the atlas, making the identification of structures much easier.
The diagram below shows the correct angles to block the brain for each stereotaxic orientation.
There is a plexiglass cutting guide available in the lab to help make the cut at the correct angle for the de Groot orientation. There is also a brain blocker available for Skull-Flat blocking, but this can usually be accomplished free hand.