Rehoming 1 yr old female cat

[Beatlelvr]

Need some advice please. My niece needed to rehome one of her cats and I volunteered. Already have 9 yr old female cat. I have new cat in a bedroom for 4 days. She's friendly, but both she and my cat know there's another cat on other side of door. Should I take her out and let my cat sniff around that room, or should I lock my cat up while new cat sniffs out rest of house? Or both? When should I introduce to each other? Also have 19 yr old female cat but she doesn't get along with anyone, and pretty much sleeps all day.
Thanks for any help.