As you know that there are two carbon atoms in the primitive cell of graphene sheet, if you add one hydrogen atom on the top of one of carbon atoms in the primitive cell of graphene and add one hydrogen atom just below the other carbon atom in the primitive cell of graphene, you will get an initial structure of graphane (i.e., fully hydrogenated graphene). Then if the geometry optimization of the lattice constant and the atomic positions is done, the optimized structure of graphane could be obtained.
