Set up a Separate Safe Space
This gives the newcomer a place to listen and smell and get used to the new home. Moving cats to a new location is stressful in itself, and takes time for the cat to adjust, let the new cat get settled before attempting an introduction; this may take a few days or even weeks.
This room should contain a litter tray, food, water, scratching post, toys and a few hiding places (high hiding places are really good). Feliway diffusers can help too.
Leave the carrier in the room with its door open so the cat can stay in there if she chooses before venturing out into the rest of the room. If the newcomer kitty is timid or fearful, being able to stay in the carrier containing her own familiar scent may provide much needed comfort in the beginning.
You are going to spend time in here making sure you get to know the new cat too.
For a fearful cat, set up some cardboard box tunnels as safe ways for her to get to resources. That way, she can get to the litter box or food without feeling too exposed.
By now the resident will be aware of the fact that the new cat is on the other side of the door. They might spend quite a bit of time looking at and sniffing at the door and that is OK.
For the next steps, read our Cat introductions - Part 2, click here