Kitchen cabinets can make a comfortable bar. Keep in mind that your bar won't be the standard 42-inch height of most bars. Standard kitchen cabinets are 34-1/2 inches high. Custom order a taller cabinet to accommodate bar stools if you don't mind dramatically increasing the price of your project. If you are trying to save money while building an attractive bar, use kitchen cabinets; the only difference will be the height of the bar, which requires counter-height bar stools.
Mark the spot where you'll install the bar. Set the cabinets in place and put tape around the perimeter of each cabinet to mark its spot. Lift and move the cabinets.
Assemble the cabinets as they will be installed for the bar. Open the cabinet doors and attach the cabinet pieces together. Drive deck screws through the side panels, attaching it to the panel that will sit next to it.
Spread construction adhesive on the back of the end panels and press them into place. The sides and back of the cabinets probably are unfinished; use end panels to create a more finished look. Let the adhesive dry.
Measure and cut four 6-inch pieces of two-by-four and set them on the floor just inside each corner of the tape mark. Measure and cut four more 4-inch two-by-fours and place them to the pieces already set, forming an L-shape in each corner. Drive 3-inch deck screws through the lumber and into the floor.
Lift and place the assembled cabinet so it fits snugly over the lumber at each corner. Drive screws through the kick plate into each piece of lumber to secure the cabinet to the floor.
Cut a piece of plywood that matches the size of the cabinet with a circular saw. Drive 3-inch deck screws through the top of the plywood and into the cabinet frame every six to eight inches.
Run a bead of construction adhesive along the top of the plywood. Set the counter in place. It should be at least 10-1/2 inches wider than the cabinets to accommodate a chair and someone's legs comfortably.
Clamp the counter to the plywood with c-clamps, and let the adhesive dry overnight.