I do this on a weekly basis, and this is how.
First, I'm assuming that you're doing this on live, anesthetized, albino Sprague-Dawley rats. (If you work with white-furred, red-eyed rats, then chances are you are working with albino Sprague's.) Second, I'm assuming that you have the following tools:
Right then. First, you should decapitate your animal. Do it quickly and cleanly, and make sure to make the cut as close to the skull as possible. You'll be left with a (dead) rat body and head.
Begin by exposing the skull. Using your scalpel or razor, make a single cut along the length of the head, starting at a point between the eyes, continuing along the dorsal (top) of the head, all the way through to the posterior aspect (back end) of the head. The skull will be covered by a layer of skin and a layer of cutaneous muscle, so you may wish to do this cut twice -- i.e. once to cut through the skin and then again to cut through the muscle. Using your ronguers, pull back the two flaps of skin and muscle to expose the skull underneath. You may need to scrape stray bits of muscle off of the skull surface with your ronguers or with your blade. In addition, there might be quite a bit of thick neck musculature surrounding the base of the skull; you should pull or cut this away so that you can see the entire dorsal/posterior aspect of the skull, with the base of the brain stem visible at the posterior-most point.
Now that the skull is exposed, it can be removed. (This next bit is tricky to describe, so bear with me here.) For this, use the ronguers to "peel" bits of the skull away from the brain surface. Start by placing one "scoop" at the opening at the base of the skull, and the tip of the other scoop at a point dorsal, anterior and somewhat lateral to the first scoop; using this point as an anchor (i.e. the scoop stays fixed to this point) pull the ronguers closed so that the other scoop pulls and breaks off a little bit of skull from the base of the brain. Continue in this way to remove little bits of the posterior portion of the skull. At some point, you may find it helpful to move your anchor point laterally, almost to the ears, and thus pull the skull off in two halves from either side. You can also give yourself a faster start by inserting the sharp tip of your large shears underneath the base of the skull (about 2-3mm) and actually cutting the skull dorsally along the midline -- though this may damage the cerebellum, which lies directly beneath this part of the skull.
After removing the posterior part of the brain, the rest of the skull should come off rather easily; again, use one scoop of the ronguers as an anchor point and use the other scoop to reach under the skull and pull it up and off. I find that after removing the posterior skull, the dorsal portions come off in three large pieces: one small piece extending from the posterior-most part of the remaining skull to the lambdoid sutures, and then two large pieces composed of the left and right half of the remaining skull which extends to the coronal sutures.
At this point, there will be some skull left which is anterior to the coronal sutures, and you may wish to remove some of it in order to make the brain extraction easier. As before, use your ronguers, but be sure to use short, controlled pulls so as not to damage the brain underneath.
Still with me? Now that the majority of the skull is gone, you'll be able to see most of the dorsal aspect of the brain. You should notice over most of the brain surface a tough, shiny membrane called the dura mater. You'll need to get rid of this "dura" before removing the brain. The most simple way I know of is to use your small, fine shears; at the posterior part of the brain (i.e. the cerebellum) you'll be able to put the tip of the shears underneath the dura and, using a gentle lifting motion, make a single cut from back to front. Use the ronguers to pull each flap of dura to the side and expose the surface of the brain. Take care not to pull to hard with the ronguers, as you can tear the brain if you use too much force.
Finally, gently, place the blade of your scooped spatula between the brain and the dura on the lateral side of the brain and push the blade down along the side/bottom of the skull. Be very careful not to turn the blade into the brain! The idea is to place the spatula between the dura/skull and the brain on the very bottom, so that you can then slowly and gently lift the brain out of the skull. I find that it is helpful to point the head upwards (i.e. nose in the air) and to let gravity help the brain out. Note that you will have to use steady, gentle force as the brain's connections with the olfactory bulb and cranial nerves will provide some resistance.**
When free of the skull, the brain will still not be 100% "out": it will still hang loosely out of the back of the head by two remaining cranial nerves. Use your spatula to pull the brain free from these connections, and the job is done. You should preserve the brain in paraformaldehyde or formalin ( or formaldehyde if you have it), though 90% ethyl alcohol will do in a pinch.***
* These are hard to describe if you've never seen them. Imagine a pair of needle nose pliers with the nose bent at a 15 degree angle (out of the plane of the pliers) and the face of the pliers hollowed out -- it is essentially a pair of "scoop-faced" pliers.
** Also note that as you lift the brain out, the manual stimulation of the cranial nerves may cause the muscles in the ventral neck and jaw to spasm quite a bit. When you're holding the head in your hand, this is -- far and away -- one of the creepiest feelings ever.
*** That's it. This is the end of the writeup. Ok?****
**** Look, what is it that you want? The writeup is over! Go, get out of here! Go read about lesbians or something. See here: if all you want is some nauseating, puerile rant about the shameless defiling of animal parts, then you're just going to have to go and find it somewhere else!