HELP!!!PLEASE!!

OK so I have a kitten I got about a month and a half ago, she is currently 14 weeks and is the sweetest baby but she does have separation anxiety for me so I thought it would be a good idea to get her a little kitten friend. I decided to get a kitten last night who was nine weeks and I made the mistake of trying to introduce them right away. Please give me advice on how to make them get along, my cat that I already had is very territorial and jealous and she hissed at her and hit her in the head with her paws. The nine week old kitten is very scared! I have her in the restroom secluded by herself with all of the things that she shall need. But any advice on how to introduce them and when?