Measure the space that you have set aside for the bar. Write the length and width of the area on a piece of paper.
Sketch the outline of the completed bar onto the paper with measurements.
Place the cabinets in the designed layout. Screw the cabinets into place by screwing through one cabinet into the other with the wood screws. Space the screws every six inches along the area where the cabinets meet.
Measure the top area of the cabinets to cut the top of the bar. Transfer these measurements to a piece of plywood or MDF. Add one foot to the outside edge of the top for an overhang. This will give you room for people's knees and legs when sitting at the bar, or space to store stools. Cut the pieces with a circular saw; for corner cuts or cutouts, use a jigsaw.
Place an L-bracket against an inside edge of the cabinet and attach it with a screw so the bottom of the "L" is upward and can attach to the underside of the top. Space the brackets evenly around the inside of the cabinets every four to six inches.
Place the plywood on top of the cabinet base, and attach to the "L" brackets with screws. Line the edge up with the side of the cabinets that have doors and allow the excess to hang over the backside of the cabinets.
Sand the top of the bar beginning with 100-grit sandpaper and increasing to 220-grit sandpaper.
Wipe the top of the bar with a damp cloth to remove all dust and debris from sanding. Allow to dry.
Paint or stain the top of the bar as desired.
Seal the bar with three coats of clear polyurethane, allowing each coat to dry thoroughly before continuing with the next coat.