@heolyheather — scent swapping, (putting their beds in each others areas), allowing them to explore the others area (eg: new cat exploring the house while the current cat is locked in another room new cat hasn’t got access to), visual contact ,
crates can also work, put one in the corner of the room and cover it partially with a blanket to allow it to feel safe and be able to go out of sight. allow for the other cat to roam the room while the new cat is in the crate,
hope that somewhat helps