Introducing a new cat to an existing cat can be a bit tricky, but with some patience and planning, it can be done successfully. Here are a few tips to help make the introduction go as smoothly as possible:

1. Gradual introduction: Gradually introduce the new cat to the existing cat by first allowing them to catch each other’s scent through a closed door. You can do this by swapping out their bedding or rubbing a towel on one cat and then the other. This will help the cats get used to each other’s scent before they actually meet.
2. Separate spaces: Set up separate spaces for each cat, with their own food, water, and litter boxes, so they each have their own territory. This will help them feel more secure and less territorial.
3. Controlled interaction: When you do introduce the cats face-to-face, make sure it’s in a controlled environment, such as a large room with plenty of hiding spots and escape routes for either cat. Keep the first few meetings short and supervised.
4. Reward positive behavior: Reward positive interactions between the cats, such as if they sniff each other without getting aggressive, with treats or positive reinforcement.
5. Give them space: During the initial introduction period, give the cats plenty of space, and don’t force them to interact. If one cat seems uncomfortable or aggressive, separate them for a while and try again later.
6. Keep up the routine: Keep both cats on the same feeding and grooming schedule, which will help keep their routine stable during the transition period.
7. Be Patient: it’s important to be patient throughout the process, as it may take weeks or even months for the cats to fully adjust to each other.

It is also important to note that even with a successful introduction, some cats may never become close friends and may simply learn to tolerate each other’s presence.