Tie the flap up so that the cat can go in and out (without it going down).
Gradually lower the flap but still keep it tied up. Also, place food inside
so that the cat can see and smell it. I have the same flap.
Gail

If your moggie wants to go out, it will.
For the first couple of weeks I used sticky tape to stick the flap open
whilst I was in and Buffy would sit with her nose out having a sniff. So, I
found that if I went out into the garden on nice evenings with a coffee and
just hung around, she was nosey enough to come out and see what was going
on. Coming back in wasn't really an issue, if I came in, as soon as she
appeared at the flap I would open it slightly from the inside and after a
few attempts at opening it with her paw, she soon worked out she could do
eassier by pushing with her nose.
But, after a lot of expense (I had a new window installed so I could have a
flap) and effort, my moggie has decided in her infinite wisdom she is
mainly an indoor cat and the occasional sniff out of the flap is enough for
her!! Cats....don't ya love 'em :-)

My two older girls took almost a week to open the flap.  It's a screened
flap attached to my sliding screen door to my deck.  Eventually when they
saw me sitting out on the deck, having a snack, they pushed their way
through.  The kitten I got at 8 weeks old took about 2 or 3 days to get the
hang of it.  She watched the older ones go in and out and she went right
ahead first using her paw to push out the door.  I then showed her to just
use her head to push her way out.  I think a screen may be easier to use
than a plastic flap.

My cats do not go off the deck which has a 6 foot high fence around it.

Barb