Couple of thoughts from someone who has done this once or twice but it by no means and expert, or is even qualified as being moderately successful at their own attempts. First expect that your slab will develop some warp, twist, and likely some checking/splitting as it dries. This the nature of the beast, the amount is dependent on a lot of factors including cut, species, thickness of slab and environmental factors as it dries.
To minimize end grain splitting I would recommend coating the ends of the slab with something to slow and control water loss from those surfaces. Latex paint is a cheap solution, paraffin wax is another, and anchor seal is a purpose made product you can use. If you have the ability storing your slab some place level and out of the elements would be best. An insulated garage, climate controlled basement, etc. would be ideal. What you are trying to do is provide a relatively constant environment within which the slab's moisture constant can reach a stable equilibrium with the surrounding environment. Drastic fluctuations in temperature and humidity are not conducive to this. This will likely take a while. I had a 2.5" eucalyptus slab that took at least a year to fully dry.
I would also recommend storing you slab so that it is elevated off the ground so that air can circulate across both surfaces. Setting on a pair of saw horses, or even couple of stickers so that it isn't resting on the dirt or a concrete slab will accomplish this purpose.
As for getting smooth surfaces....wait until your slab has dried! I know its tempting to level out that slab now, but you're going to have re-plane it once it has dried so you might as well wait and do it once and not remove more wood than necessary. As for planing it you might be able to find a local mill to do it for you, or if you have access to a router you can make a large sled and do it yourself. When I planed mine down I screwed a couple of 8' long section of 3" PVC pipe to my work table and mounted my router to a plywood sled and did a rough planing in that matter. It worked surprisingly well. Google router planer and the images that come up will give you a pretty good idea. In any case good luck, hopefully someone more experienced than I will be along shortly and give you more detailed advice.
