I absolutely agree with the prior posters. As long as any of the cats is unaltered, your introduction will be complicated. Also, when Misto goes into heat, she will start attacking your living room door for other reasons, she and Thomas will become "best friends", and a few months later you will have cats 5, 6, 7, 8, 9, ... to deal with!
So, step #1 is a vet visit.
As for step #2...a few questions. How old is each cat? How long have the cats been together under the same roof? Aside from giving Misto her own room, what else have you done thus far to introduce the cats?
In the meantime, I suggest you hold something (such as a pillow) close to your feet/legs when you go in and out of the living room, to prevent Misto from accidentally getting out again. If she tries to sneak by, you can "shoo" her back in with the pillow...or, at the very least, the pillow will slow down her escape and permit you to nab her.