You can use a skill saw or a 4 1/2" angle grinder with a diamond blade, along with a straight edge.
Set the saw blade if using a saw, to about 1/16"
Measure your lines out, mark each end with a pencil, and then mark the lines with a chalk line, using blue chalk. Never use red chalk as it stains the concrete.
Use a straight edge of some sort ( 2 x 4 board, piece of angle iron, etc.) Holding the straight edge firmly and the proper distance from the line to ensure the diamond blade is on the line, score away.
As your saw or grinder will not score completely to the wall, you will need to cut a border about 8" +/- around the walls and take your pattern scoring to the border lines.
Tip. Using a 4 1/2" grinder to score the lines, is usually much faster and you can use a flat straight edge. Also, most slabs tend to have slight high and low areas and a skill saw can sometimes skip because of this, as where a grinder will follow the natural contour of the surface.