across, a cross

Across or across ( one word ) means from one side to the other--
They sailed across the Atlantic in a great ocean liner.
He walked slowly across the room.
He folded his arms across his chest.
Draw a line across the paper, about an inch from the top.
Across may be used figuratively, meaning in terms that can be understood or that will be agreed to--
He can't seem to get his ideas across.
We'll never put this deal across.

A cross or a cross ( two words ) means one ( a ) structure with one upright piece, and two arms that project at right angles from the main part ( cross )--
He found some wood to make a cross, and used it to mark the grave.
He wore a cross around his neck, with a small gem in the center.

A cross ( two words ) may also describe one annoyed or ill-tempered person or thing--
What a cross old man!
He rarely says a cross word.

A cross may also mean one ( a ) thing or idea that is a hybrid of two others ( cross )--
A mule is a cross between a horse and a donkey.
It's a cross between pizza and spaghetti; do you like it?

Now that you know that, you can say--
"He walked across the field one way, and then the other, marking out the shape of a cross as he went."