Determine which cabinets will make up the base of the vanity and move them into the project space. Remove the doors and drawers to protect them from damage and to keep them out of the way.
Drill two pilot holes through the vertical stiles at the front of the cabinets with a countersink bit.
Screw the two cabinets together. Since the cabinets used in this project were made of very hard wood, the screws were rubbed in a bar of soap before being driven into the cabinets. The soap lubricates the screw and helps it move more smoothly. If assembling more than two bottom cabinets, attach them to each other in the same way.
There is a gap between the cabinets along the back edge. Slip shims between the cabinets at the top back edge (Image 1), then use a level to square up the cabinets.
Clamp the cabinets together around the shims (Image 2). Drill pilot holes through the cabinets and shims, and attach with screws.
With the cabinets attached, line up the cabinets in front of the wall. Find and mark the center point of the assembled bank of cabinets. Also mark the center point of the roughed-in plumbing fixtures. Use these as reference points when measuring for drains and other holes.
Measure and drill holes in the bottom of the cabinet for the water supply lines Also measure and drill any needed holes to run electrical lines to the upper cabinets.
Move the bank of the cabinets into place, lifting the assembly to thread the water supply lines through the appropriate holes. Line up the drain holes with the appropriate pipes.
Use shims to level the cabinet from front to back and from side to side. Use a 6' level to get an accurate reading for the long cabinet.
Locate the studs in the wall behind the cabinet and use a level to draw a plumb line from the studs onto the inside back of the cabinet.
Remove the shims from under the cabinets. These spaces will be covered by a faceplate that will be installed with the final trim work. Reattach the cabinet doors and slip the drawers into place.