Measure and cut a piece of vinyl-coated particleboard called “melamine” to fit the sink cavity opening and drop it onto the cleats to serve as a bottom for the form. The melamine should be flush with any existing surface on the cabinet. Its vinyl-coated surface will not stick to the concrete. Later, after the concrete countertop has set and you have removed the ledgers, you can easily remove the ledgers and melamine.

Attach two temporary two-by-fours at the upper edge of the vanity, setting them flush with the vanity’s upper horizontal surface and placing the screw holes as inconspicuously as possible. The two-by-fours will extend the bottom of the concrete form so that the countertop will have a decorative lip or overhang on completion. We wanted the overhang on the two exposed edges of the countertop only—so we taped (with electrician’s tape) the two edges that butt against the walls to seal the bottom of the form and protect the walls.

Secure a strip of melamine to each of the two-by-fours to form a 2 to 2 ½-inch wall for the concrete form. The top of this wall will mark the depth of the countertop, so ensure that the installed height of the melamine matches the desired depth of the finished countertop. If you make it too thick, the countertop surface will not be a standard elevation from the floor.

Cover exposed wooden surfaces with electrician’s tape to ensure they will release easily from the concrete when you remove the form. The tape’s non-porous suface prevents sticking and lessens the likelihood of damage to the edges of the hardened concrete countertop.

Cut a Styrofoam block the same diameter and shape as the sink to make a barrier/dam to preserve the proper opening for the sink. Ensure that the vertical sides of the foam are slightly greater than the intended thickness of the concrete countertop to prevent the wet mortar from burying the Styrofoam block. Taping the edges with electrician’s tape will help prevent the concrete from sticking to the edge of the Styrofoam and give it a little support.

Set a piece of reinforcing steel/mesh (with a cutout for the sink and its hardware) in place to give the countertop additional strength and prevent cracking—in much the same way that rebar is placed in concrete pads when they are poured.

Work the mortar to a uniform crumbly consistency with a mason’s hoe and then roll the wheelbarrow into the bathroom after protecting the floors along the route.

Mix another batch of mortar, this time starting with quick-drying cement. Before beginning, hose the wheelbarrow and other tools to remove any coarse aggregate and residue. Allow the clean wheelbarrow and tools to dry, because you will be mixing a series of dry ingredients for this batch or mortar and do not want moisture to cause lumps before you have completed the "recipe".

Add marble sand, white Portland cement, and mineral pigments—all are available from your home improvement store. Follow manufacturer’s directions in judging the quantity of these additives to give the countertop a finer, richer texture and color than the core layer of concrete you already poured. Blend the quick-drying cement and the additives thoroughly while they are dry.

Add water to the dry ingredients and mix it to a uniform consistency with the mason’s hoe. The mixture should be just thick enough to hold its shape when you scoop out a shovelful.

Trowel the concrete to smooth out and burnish the surface. Use a flat hand trowel on the surface, being careful to hold it parallel to the surface. Pull it horizontally, alternately working in small arcs, to smooth any imperfections in the surface. Be careful not to tip the trowel or gouge its corners into the concrete—filing the sharp points off the corners of the trowel blade can help. Repeat this process several times while the concrete continues to set over the next few hours.

Allow the concrete to dry overnight and remove the form—that includes the two-by-fours, melamine pieces, ledgers, and the Styrofoam sink dam.