There are a variety of ways to express the concept of "fall." It
depends on your meaning.
If you mean "fall" as in "fall down" or "fell onto the ground" you
can use a "V"-handshape to represent a person falling. You can
modify the movement of the falling process to show exactly how the
person fell (this is called "using a classifier").
FALL-DOWN

You can modify the FALL sign to mean "fall" as in
to "fall off of something."
Use the upside down "V"-handshape to "stand on the non-dominant
palm. Then show the "V"-handshape falling off the palm and downward
a foot or so.
FALL-off:


Notes:
You can do the FALL sign from the head area (hold the "V"-handshape
up near your scalp) using and moving both hands alternately (first
one hand then the other) to mean "hair fall out."
If you mean "fall" as in "fall in love" see:
FALL-in-love
AUTUMN: If you mean "fall" as in the season "autumn" then
visit the "autumn" page. See: AUTUMN
You can learn
American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars