Isolation for the newcomer supplying two litter boxes, food bowls, water dish, and toys.

Visit the newcomer often, sit on the floor and read out loud, softly.

If kitty hides, just accept it, she will come out to you eventually don't case her.

To soothe the newcomer use the alarm clock trick, or play classical music.

Use the blanket trick, it really works and makes the initial first meeting less stressful for everyone.

Add an old tee shirt that you got really sweaty to the newcomer's room.

First meeting have help and blanket or heavy towel at the ready to toss over any aggressive kitty.

Don't neglect either cat during the isolation period. Interactive play time between both to them and yourself is very important.