Lay one 30-inch board on a flat surface. Measure 10 inches from one side of the board, and draw a straight line across the 12-inch side of the board. Stand the 10-inch-wide board on top of the 30-inch board so that it is perpendicular to the board and the 12-inch side of the board lines up with the pencil line. Draw another pencil line on the other side of the 10-inch board. You have two straight lines, 1 inch apart on the 30-inch board. Repeat with the other 30-inch board.
Make three marks between the two pencil lines on the first 30-inch board, each spaced evenly apart. Each mark should be about 4 inches from the others. Repeat with the other 30-inch board. Also make three evenly spaced marks on the edge of the 12-inch sides of the 10-inch board. On the 10-inch board, each mark should be about 4 inches apart.
Drill a 1/2-inch hole at each mark. You should have six holes total on the 30-inch boards -- three on each board -- and six holes on the 10-inch board.
Put a drop of wood glue into each 1/2-inch hole. Push the wood dowels into the holes on the 30-inch board. Line up the holes on one side of the 10-inch board so that they line up with the three dowels on one side of the 30-inch board. Push the 10-inch board on top of the 30-inch board until it is flush with the board. Push the second 30-inch board onto the 10-inch board, lining up the dowels on the board with the holes on the 10-inch board.
Stand the 30-inch boards up so that the 10-inch board is parallel with the ground and near the top of the boards. Let the glue dry overnight.
Place one 12-inch board on top of the 30-inch boards, lining up the edges to form the top for the table. Drill three screws into each side of the 12-inch board to attach it to the 30-inch boards. Space the screws evenly, about 4 inches apart.
Turn the bedside table upside down, and attach the remaining 12-inch board to the 30-inch boards in the same way you attached the top. Turn the table right side up again.
Apply a coat of paint or wood stain to the bedside table with the paintbrush. Let the paint or stain dry for 4 hours or overnight, then apply another coat.