One side of my slab was smoother than the other to start out, so I picked the smoother side of the wood to start sanding. The underside does not need to be sanded, but it should be relatively flat.
I started with a rough grit of 60 or 80 and worked to smoother grits until it was satisfactorily flat and smooth.
Wear eye protection and a dusk mask while sanding, and hearing protection if your sander is loud. Sand outdoors if you can!
Sanding will take a long time. Be patient and don't forget to take breaks!