There are many possibilities for finishing the counter top. The simplest is to do no grinding at all but to simply slurry in any pinholes or voids that there may be on the top surface of the counter top and then to polish the concrete starting with a 200 grit diamond polishing pad.
The other option is to grind the surface to expose the aggregates (rocks and sands) within the mix. By grinding with a 50 grit diamond polishing pad the larger aggregates can be exposed. By starting with 100 grit, the sands can be exposed. It is a matter of taste as to what looks best. With a practice piece it is possible to try different styles in different parts of the same concrete slab.
If the decision is to expose the aggregate, start the wet grinding two days after the slab is flipped out of its mold. If the piece will only be polished wait for day five.
BE SURE TO DENSIFY PRIOR TO 100 grit pad, CLICK: DENSIFIERS
All grinding is done with water streaming onto the counter top. There are a few ways to accomplish this. If a water-fed grinder is not being used one method for keeping the surface water fed is to poke holes in the bottom of a large plastic container (a large vinegar container works well), place it on the counter top and feed a hose into the opening with the water trickling in. A constant stream of water will wash out of the bottom holes onto the counter top.
For exposing the rocks, work slowly with a 50 grit circular diamond pad attached to the grinder across all areas of the surface until the desired aggregate exposure is obtained. Switch to a 100 grit pad and repeat the process until the desired finish is achieved. Hose off the surface well. Slurrying can be done the next day.