Sculptors marked a grid on a block of marble and drew the front, sides, and back of the kouros to proportion on the grid. They used a punch and then chisels to chip away the extra stone and then a rasp to finish the product. It has been said, with good reason, that to walk around a kouros is to walk around a corner.

This diagram is from S. Woodford, An Introduction to Greek Art
(Cornell University Press, 1986)