Letting them see each other is a good start. Scent swapping and location swapping would be next. Letting them sniff each other through a crack in the door would come after. Shared treats, play time, and meals next, working up to shared spaced 100% of the time. Give lots of praise and love to all.
In my opinion, people seem to think introductions have to be a all or nothing sort of thing and that is how it shouldn't be. Just like people irritate each other, animals do the same and need breaks from each other and time to process and relax.
Expect some growls and hisses both at each other and at you. Time and patience will help a lot.
Kudos to you for fostering!