Bought a Savic Nestor Cat covered Litter Box as the 9 week old kittens are getting bigger, current open litter trays will be too small very soon.
I've placed them inside but they are not too sure on the purpose of the door flap, I think it might scare them a bit, they think they have been locked in

. I have to open the flap to let them out. Any tips/tricks on getting them to use the door on their own, or is it a case of perseverance and patience?
I put some litter in the new box from their old trays so they recognise their scents.