Measure each side of the countertop.
Install cement board over the cabinets as the bottom form of the concrete kitchen countertop
Cut 2x pieces of wood the size of each side of countertop that will need formwork.
Cut the melamine plank the size of each side of countertop that will need formwork.
Attach the 2x pieces of wood to the cabinets with wood screws long enough to go through the 2x and embed into the cabinet. The 2x pieces should be level with the bottom of the countertop. This allows for the countertops to overhang the cabinets the width of the 2x piece of wood.
Attach the melamine planks to the 2x pieces of wood and set at the height you want the countertop to be.
Measure the total length and width of the countertop.
Cut the 3/8-inch rebar into pieces that are approximately 1/2 inch less than the overall length and width.
Lay out the rebar inside the formwork, placing the rebar every 6 inches and around all of the perimeter edges to create a grid of rebar inside the formwork.
Tie the rebar together with the wire ties so they act as one solid piece.
Lift the rebar so it is floating and located approximately in the middle of the concrete kitchen countertop.
Tie the rebar at this height with the wire ties and screw it to the side of the formwork to hold it in place. You can also make small concrete chairs to hold the rebar in place while pouring the concrete.
Mix the concrete per the manufacturer's instructions inside the concrete mixer. It should be the consistency of oatmeal.
Pour the concrete on to the kitchen countertops, a little at a time.
Push the concrete under and between the rebar, using your hands if necessary.
Vibrate the concrete using the vibrating machine every time concrete is added to the kitchen countertop. This will help reduce voids that can occur inside the countertop.
Finish the Concrete
Use the wood float to level the concrete after you have poured enough concrete to reach the top level of the formwork.
Lay the wood float on the formwork and screed it across the concrete kitchen countertop to level the concrete.
Check the countertop with the level to make sure it is flat.
Trowel the concrete with the steel trowel once you have leveled it. If you trowel the concrete and it brings up water, wait for a few minutes to let the concrete harden.
Trowel the concrete continuously. This will give it a smoother finish and will require less grinding and polishing.
Cover the concrete with plastic to let it cure.
Removing Formwork and Polishing
Remove the formwork gently after the concrete has hardened.
Polish the concrete kitchen countertop with a diamond abrasive around the edges to remove some of the excess concrete.
Polish the tops of the concrete kitchen countertops to expose more of the aggregate. How much you polish will depend on what kind of look and style you want the concrete kitchen countertops to look like.