The sign "across" has several
variations depending on your meaning.
The generic sign for "across" uses the "OVER" sign. To do this sign you
use a "flat" hand shape on your dominant hand and slide it across the back of
your (palm down) non-dominant hand.
You can learn sign language online at American
Sign Language
(ASL) University (tm)
Sign language lessons and resources. Dr. William Vicars (c) Lifeprint.com