A whole beef brisket is oblong, or oval shaped. If you have a point cut or flat cut half of beef brisket, they are still longer than they are tall.

With the brisket laying in front of you, with the longer piece laying left to right, cut straight down from top to bottom.

This will cut the brisket across the grain.

Another way to visualize it is to compare the meat to a piece of wood board. Usually you will notice the "wood grain" running in a long line.

Beef, or any cut of meat, has muscle fibers that run in "one" direction also. Just cut perpendicular to this long fibers of meat.