Randy, look at my post under building a cement sidewalk that is down lower a ways. That will give you some guidance except a patio slab is much larger and you may want it a little smoother than a brush finish.
For a patio slab, you will want at least 3 people to make the pour and somebody that is experienced in finishing concrete, trowling, etc. You probably don't want to rent a power trowl for that perfectly smooth finish or else everyone will be falling on their rears when it rains or you get a little ice on it. Post back if you have something specific. Good luck.