Cut a 12-by-11.5 inch piece of 3/4-inch MDF (medium density fiberboard). This will be the seat.
Cut a 1-by-4 board into the following lengths: 11.5 inches (2) and 13 inches (2). These will be the seat sides.
Cut 2-by-2 boards into the following lengths: 30 inches (4), 10 inches (4), 13 inches (2). These will be the legs and supports.
Place the MDF seat on a work table.
Position the two 11.5-inch side pieces next to the 11.5-inch edges of the seat. Drill three pilot holes through the side and into the seat. Connect each side to the seat with three 2-inch wood screws and wood glue.
Position the 13-inch side pieces on the other sides of the seat. They will extend across the seat and 11.5-inch sides, creating a box. Glue and screw these pieces to the seat. Add an extra screw into the side of each 11.5-inch side piece.
Build the first leg frame by making a rectangle out of two 30-inch 2-by-2 boards and two 10-inch 2-by-2s. The 30-inch boards are on the outside of the rectangle, sandwiching the 10-inch boards between them at the top and bottom.
Drill pilot holes and connect the frame with 3-inch wood screws and wood glue.
Build the second leg frame following the instructions for the first.
Set the two leg frames on the 11.5-inch sides of the upside-down seat. Drill pilot holes and screw them into the seat with 3-inch wood screws. Use wood glue as well to really secure them in place.
Flip the bar chair over onto its legs.
Glue and screw a 13-inch piece of 2-by-2 between the legs at the front and back of the chair, positioned at the floor.