Introducing a soon to be former stray cat to my house cat

[mr_lebowski]

2. Put the new cat in a room that the others don't commonly use (if possible)

Give her her own litter box in there of course.

The others will come round, they'll smell each other under the door and generally get used to each other, probably paw at one another under it.

There should be lots of advice online about introducing new cats to an existing cat household. But I've done it before as described above. Obviously if they seem to be playing footsies under the door and not hostile instead, this is a good sign.