Start by bringing the cats into the same home, but keep separate litter boxes and food and water bowls for at least a week. You can allow cats to sniff each other under a door to help them adjust to each other’s scents. Then, you can allow them to see one another, but try to keep them apart physically with a mesh screen or in two separate crates facing each other. Allow them to come close to their respective sides of the screen for enjoyable experiences, like eating food or treats, or playing with toys. Finally, if they show relaxed body signals, you can introduce them without a barrier. Just make sure they are supervised and each has their own spaces to retreat to if they need a break.