The sign for "flat" has many interpretations depending on the rest of your sentence.
FLAT (GENERAL)
Also means: "floor," or "carpet" (in context), or even "shelf."
FLAT-SURFACE:
Use "bent hands." The dominant hand moves forward, the non-dominant hand stays stationary.
FLAT-TIRE:
[This sign also can mean "deflate."] Use a quick motion unless you are trying to indicate that something went slowly flat.
Notes: