I have an older home (1920) with 2 bathrooms missing fans. What I did
was to get the fan/light combo and cut out the ceiling from the fan to
the wall. I then cut through the outside wall, just above the ceiling
to install a vent. Connected the vent to the fan with a flexible metal
snake tube (think dryer vent). After, patched up the hole in the
ceiling and put a coat of textured ceiling paint over it (hides the
ametuer job of patching). Connected the fan/light to same switch, so
both fan and light come on when switch is turned.

Keeps the bathroom much dryer after showering. Took abolut 2 days to do
each bathroom, between the installation and painting.