Follow the standard fitting instrutions in these Youtube links
https://www.youtube.com/watch?v=QayPUvuF8xM&ab_channel=SupertrimSE
https://www.youtube.com/@SupertrimSE/videos
If the cushion is removed from the seat then pull the dowel from the back as far as possible ( this is best done with the chair adjusted to the most acute angle to allow for the gap to open up slightly more and let the dowel through). After this make sure to fold the cushion inside out and stretch it over the seat. Finally, lay your hand flat against the gap at the front of the seat and try to tuck any excess materiel towards the same gap and it should help achieve a much better fit. (In the dowel video I fit the cover using this exact method)
If you have followed this method and are still experiencing issues, we could add Velcro straps which connect from the dowel to the front cushion section. This will help hold the cover tighter and give it less of a chance to shift on the seat.