Kitchen island cabinets are actually easier to install than the cabinets attached to walls. Some of the same principles apply, like how to attach one cabinet to another, but the process of attaching the cabinets securely to the floor must be done differently to ensure your island does not move. Follow some simple guidelines and this project should go smoothly.
Mark cabinet measurements on your floor with a chalk line or pencil sketch to ensure proper placement. Give adequate clearance to surrounding walls, appliances and cabinets---3 feet at minimum.
Measure the underside of the cabinets for placement of wood blocks or cleats. Calculate the thickness of the toekick. Mark the measurements on the floor of the inside cabinet perimeter inside your chalk lines, allowing enough extra thickness for the toekick measurement.
Place 2 by 4 inch wood blocks that are 8 to 12 inches in length on the inside toekick measurement. Blocks should be positioned at both ends of the island and at the front and back; you may require two or three on each side. Secure wood blocks to the floor using either nails or screws, depending on your floor type.
Place the cabinets over wood blocks. Connect the cabinets tightly together and place shims, or little scraps of wood, underneath them to make them even; check by using a level. They should be level on all sides and the front faces of the cabinets should be plumb, or even, with each other.
Use two clamps to lightly hold the cabinets together, with clamps placed above the drawer slide and above the bottom hinge. Recheck front levelness then tighten the clamps.
Use a 1/8 inch drill bit and set your drill at 2 ½ inches of depth. Pre-drill a hole, without the screw, from the side of the cabinet where the hinge plate sets, on the inside of the frame starting at the top hinge area (the cabinet door should be off the cabinet for now). Hold the drill against the top of the cabinet opening and drill your hole 2 ½ inches into the first cabinet and slightly into the adjacent cabinet. Take a 7/32 inch drill bit to make the hole a little larger, which will provide a tighter connection and prevent wood from splintering.
Pre-drill a matching hole from the bottom of the cabinet opening above the hinge plate area by holding the drill tight against the cabinet opening. Realign and level, if necessary, and drill your screws into the holes tightly.
Secure the cabinets to the wood blocks on the floor by hammering small finish nails every 6 to 8 inches through both the cabinet and the block. Nail the toekick in place at the cabinet base. Screw and set the cabinet doors and drawers in place, adjusting as needed for levelness.
To readjust placement of the cabinets for alignment once loosely clamped together, a rubber mallet and/or block of wood covered in a soft cloth or towel can help position the cabinets correctly without scratching the wood finish. Paraffin or bee's wax rubbed along the twists of the screws makes the drilling smoother and easier. Pre-drilling the hole also helps. Using both of these methods will help prevent cabinet wood from splitting.