Well first buy The Traditional Bowyers Bible Vol 1, And then if you haven't already, split the log. and seal the ends with some white glue to avoid cracks.
You can either leave the sapwood and the bark on and it will take a little longer to dry, would definitely be ready by august though, or you could remove both and get down to the heart wood and it would be ready around the end of april or early may.
Be very carefull when "chasing rings" as if you dig into the next ring there is a very high chance you will get a back plinter or your bow will sudenly become much shorter.
Again reading TBB 1 will tell you almost everything you need to know. Also take 10min and get an acount on Paleo Planet. There are a few primitive archers on here like myself, Bill Skinner, and David Morningstar, but there are dozens of primitve archers that have lots of expierience on osage bows there (including us three, my username is Woodland Archer). Either way you also might want to try making a Red oak board bow first just to learn the way wood works and to get some expierience before you dig into a good osage stave (It will probably break or be underweight after many tiller adjustments, but even that's part of the learning process to help you make better bows in the future.). You will learn a LOT with your first few bows so it would be really nice to have at least some of that knowledge when you make that osage bow.
Whatever happens, don't give up, primitve archery is no doubt one of the most difficult paths to take but it's also the most sceneic and the most rewarding.