When I fitted C/F I wondered how the dickens I'd teach half dozen or so cats
(own, strays, visitors) to use it (used to leave kitchen window open). I
grabbed the smallest female (own), tried to push her through, she looked at
me for the next week like I proverbally 'ate Kittens for breakfast'.

I took the appliance off, leaving the hole for a week, put the frame on for
week, etc
Eventually all worked out much to my relief and allmost surprise.

Everycat comes an goes as they please now, except when one wants out and
another wants in, they'll stand thiere glaring through the flap at each
other.