Draw two 12-inch lines across the board parallel with the 12-inch edges, spaced four inches apart, and each two inches away from the 12-inch edges. Draw two lines perpendicularly across the first pair and space them six inches apart so they are three inches away from the 8-inch ends of the board.
Drill a 1-inch deep, 3/8-inch hole where each of the lines intersect, and erase the remaining lines.
Place a dab of glue inside each hole, just enough to cover the bottom of the hole. Place the 3-inch dowels in a pair of holes that are both two inches away from the 12-inch edge of the board. Place the other two dowels in the other two holes. Let the glue dry according to the instructions.
Sand down the board and the dowels with your medium-grade and then fine-grade sandpaper. Wipe the wood slab stand down with your tack cloth, and apply your wood stain. Let it dry according to the instructions.
Set your wood slab on the board so the front of it rests against the 3-inch dowels, and so the back of the slab leans against the 8-inch dowels.