Before setting the stones, you need to make a guide. To do this, take a piece of 2-foot square plywood and center it on the top of the blocks.
Chip off the edges of the field stone with a masonry hammer. Flatter edges mean that you can square off the corners and the stones will set closer to each other, requiring less mortar joints.
Note: Field stones are good to use for this type of project because they can be notched down to various size and shapes.