Good advice from Gill, especially about the "not rushing" things. And use food as an incentive. Feeding either side of a closed door helps them to get used to each other's scent whilst they're eating - one of the most important moments in the day for a cat. It works on the same principle as the scent swapping Gill mentioned.
Mutual play as well, once they're both able to be in the same space together without any fisticuffs or hissy fits.
Continue to make a fuss of Sadie, and to show Sadie that there is more than enough of everything - love, beds, food and comfort - to go round both of the cats and to spare.
Also, two plug ins - one upstairs and one downstairs - Pet Remedy is great. They'll help. You can also buy a spray, which you can use on bedding, furnishings and your hands, which will help to calm them.
You can also buy a very safe food supplement called Zylkene, which you sprinkle on the food, or mix in with cat specific milk. They seem to love it, and that too has a calming effect. I would use it on both cats, to get the best benefit.
(I love that little guinea pig from the Secret Life of Pets
We had a white guinea called Snowy who once won us a tea set in a school raffle
)