I don't want to discourage anyone, but if you have never surfaced even smaller panels or boards I would seriously caution against starting out with a slab of any type of expensive wood, especially walnut. It took me months before I felt comfortable hand-planing large surfaces. A surface of the size your talking about will be a major chore even for those that do it regularly such as myself. My main problem is my arthritis. Anyway, back to your question:

If you want to do this by hand, here is what you are up against. There are many ways to do this but this is how I do it. If the wood surface is rough, you will typically start with a scrub plane. The scrub plane should be tuned with a 'camber' on the edge of the iron to help hog away a lot of material. You will use the scrub in diagonals across the width of the panel, first in one direction and then in the opposite direction. This removes all of the rough surface. Next, I go to a Stanley #5 jack plane and again, diagonally, work on bringing the surface to a level plane in all directions. You will constantly be checking with a good straight edge width-wise, length-wise and diagonally in each direction. When you get the surface down to flat, then I move to the smoother plane (like a Stanley #4). I begin working straight with the grain to smooth out the surface. Depending on how you're sharpened the smoother, you may have ridges left after each pass of the plane. It is a repetitive process to bring the surface closer and closer to true flat. Next, depending on the grain, you will more than likely need to switch to a scraper. For this I go to a Stanley #80 that is tuned properly. During all of these steps you are constantly checking flatness with a straight-edge. Myself I have to stop numeroud times because it is a very tiresome process but I love doing it. My suggestion since this will be obviously a very nice oiece of walnut, I would take it to a planer first. Then, I would work the surface with a scraper such as the 80 or with a scraper plane. Strongly consider this because when you're first learning to use planes, you will dig into a lot of surfaces just tuning your plane. Every time you do that on the surface of the slab you'll have to plane, sand or scrape down past that and it will feel like you're in a swamp with alligators.