It depends on the camera, but on most of them you open the back
by pulling up the rewind crank or flipping a lever, put the film in
the film well, push the rewind crank down into the film spool, pull
the leader across...here it gets different: manual focus cameras
make you put the end of the leader into a slot in the takeup spool,
and autofocus cameras have you draw it to a mark and the camera
attaches it to the spool...then you close the back and advance the
film two or three frames.
Leica M's were different: you took off the baseplate and put the
film in from the bottom.