Jun 27, 2019 · Cut a 2×12 to length (or splice it with a cleat) for one side of the ncrete for and nail it to a rner stake. Level the 2×12 and nail it to the sOkaynd rner stake. Start by choosing straight form boards. For a 5-in.-thick slab with thickened edges, which is perfect for most garages and sheds, 2×12 boards work best.