Measure the countertop area precisely. Make a diagram of the countertops, noting the dimensions of each side and end.
The concrete forms are made of 3/4" melamine boards. Melamine is readily available at home centers and lumber yards, has smooth surfaces that don’t imprint on the finished concrete and won’t de-laminate.
Cut one piece of melamine the same size as the finished countertop. On the table saw, rip four strips of melamine 2-1/4" wide and long enough to form the sides and ends of the form. The pieces for the ends should be long enough to lap the side pieces when assembled (about an extra 1-1/2", or ¾” per end, longer and wider than the countertop itself). Use screws to assemble the form.
To keep black caulk from the rest of the form, apply painter's tape on either side of the inside joints of the form. Apply black caulk to the actual joints to form a seal in the joint, and smooth it out with your finger to remove any bumps. Remember: Any bumps or indentions will affect the finished countertop. Remove the painter's tape, and check for any areas missed with the caulk, and repair them carefully.
Steel mesh adds strength and prevents cracking. (If you cannot find steel mesh, you can use ¾” rebar set in a cross pattern and secured with steel wire.
Thoroughly clean the steel mesh using steel wool and acetone. Lay it over the mold, and using bolt cutters, cut it to shape, staying at least 1 inch from the edges of the form all the way around. Drill screws every 4-6 inches around the top of your mold; you can suspend the steel mesh from these screws after you've poured in half of your concrete.
Read the manufacturer’s instructions carefully as slight deviations can and will affect the final product. The real danger is adding too much water or not getting the colors exactly right. Be sure to know exactly how much cubic footage area you have to fill with concrete. Mix accordingly.
Before mixing the concrete, put on safety glasses, a dust mask and old or protective clothing; the mixing process gets really dusty — and assemble the bucket, concrete, water, pigments and mixer.
Start to mix the concrete with water, alternating the mix and water until all of the mix has been added. Carefully follow the manufacturer's guide to how much concrete you need for the job that you are doing. Precision is important. Mix the concrete for 10 minutes or according to the manufacturer's directions. The best consistency seems to fall in sheets inside the mixer barrel as it turns over.
Carefully pour the concrete into the form (Image 1). Use a float to smooth out the surface and push the concrete to the sides of the form (Image 2). Add more concrete if necessary to reach the top of the form.
Cover the concrete with plastic (without letting it touch the surface and allow it to cure in the forms for 10 days. Take care to keep the room temperature even throughout the 10 days.
Place the handheld power sander (without sandpaper) against the sides of the form to vibrate the concrete. Vibration helps to remove any air pockets in the concrete.
Screed (move a 2" x 4" back and forth across the top of the form) to remove any excess concrete and to create a smooth surface.