The appropriate sign for "corner" depends what kind of
corner you are referring to. In general what you do is you use
"flat"-hands ("B"-hands) to show a corner, (as in the corner of a
room or the corner of a city block). However, if you are
talking about the corner of a picture (on the wall) you would
instead use an "L"-hand (or two index fingers) and do the sign
horizontally (up in the air).
The concept of "corner" has other meanings that just an actual
physical corner. There is also the metaphorical meaning of "to
corner someone" as in force a person into a place or situation from
which they cannot escape or move away from.
CORNERED / to corner someone / to pin down someone (metaphorically)
such that they must listen to you: