Just brought a new cat home help??!! :(?

Today I brought a new cat home from the Humane Society, he's (Morris) a 1yr/5mo male old american short hair very mellow and friendly. I've had another american short hair but she's(MissyFuss) female for 4yrs/3months. Missy has always been a loving cat but then again she's always been an only cat. Usually when we bring a kitten or adult cat home, female or male, she starts hissing and growling. I always thought she will never get along with another cat no matter the age or gender, but this new cat (Morris) he's so cute and loving and i really wanna keep him but i don't want to put neither of them under any stress. So far I've kept both of them in separate rooms, i dont know it only seems logical to me to keep them separate so they can get used to each others scent from far away. They've both seen each other b/c missy saw when i brought morris into the house. Pretty much what im asking is how do i get both cats to get along slowly but surely? what are mistakes i should avoid doing? any precaution i should take with the cats been together? how do i get them used to each other and how long would it normally take? plss & thx u :) I dont want to keep the cats in separate rooms b/c it defeats the purpose for which i brought morris (companionship for missy :D)