1) Make a template by outlining the shape of your countertop on cardboard. The edges should be flush with the sides of the cabinet. If the counter is bigger than a sheet of cement board, mark joints on the template; they must be placed over cabinet walls. Also mark the sink outline using the template packaged with it.
2) Using the sink template as a guide, cut a plug from foam insulation at least 1 1/2 inches thick to keep the sink free of concrete. A jigsaw or bandsaw works well. Sand edges of the foam and wrap them with plastic tape.