I'm not a cat newbie and I've had lots of cats over the years but introductions have always seemed to go easily and I genuinely don't remember the early days of the spitting/hissing etc and how long it lasted so I've come here for advice!
We have a 6 month old female cat, we had always considered getting another cat at some point but decided since she's been spayed and she's still young now would be a good time.
Well yesterday we got 2 new kittens - we were only supposed to be getting 1 but ended up taking 2 so the last one wouldn't be left alone. They are 12 weeks old, but they are tiny little things and they have wonderful temperaments. Very loving, confident and vocal. Original Dcat also has very lovely temperament (usually!) but she is very unsettled by the new kittens.
We have been keeping them somewhat separate - in the past I never did the whole keep them in another room etc but we have been giving the kittens their own space to relax together and to give Dcat1 a rest. Every time she sees them, she hisses and spits, and jumps about like they are attacking her even though they are nowhere near her and aren't bothered by her at all!
I know it's only day 2 and I'm sure she will come round in time I just want to make the transition easier for her in the meantime. She's been getting loads of love/strokes, treats, catnip etc but she's just being super dramatic! 
I know the rules so I've added pictures of the two new arrivals.