Nikon FG Instruction Manual page 17

After the last exposure, the film advance lever will not
move Depress the film rewind button on the camera
base plate to disengage the film sprocket drive so
you can rewind the exposed film back into its cartridge .
Fold out the film rewind crank and turn it
gently in the direction of the arrow until you feel an
increased tension . Give it a few more turns until the
tension is gone and the crank turns freely. indicating
the film leader is rewound completely back into the
cartridge .
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