Bought kitten home 3 days ago and keeping her in 1 room downstairs. Have been gently starting process of introduction, by opening door to room when big cat is around. Same thing happens each time - kitten runs enthusiastically up to big cat, big cat yowls and hisses, and then runs away. Big cat (who normally sleeps upstairs) has now taken to hovering on the stairs to check coast is clear before venturing downstairs.
I've just plugged in a feliway thingy which I hope will help, but any other advice gratefully received. The house is big enough for them each to have their own space, so they don't have to be besties, but I don't want my older cat to be frightened to venture downstairs..