I have done it few times. Keep them in separate rooms first. Scent is very important to cats, so they can smell each other under the door. Do that for 3-4 days, or until the strangeness will become curiosity for both of them. Curiosity will eliminate aggressive behavior somewhat. After 4 days, put the kitten in your lap while your older cat can get familiar with her, and will act a little weird (scared and curious with dilated eyeballs), and will start hissing and spitting at the young one. BUT THAT'S OKAY! DO NOT FREAK OUT! It's his way of telling the young one who the boss is! Cats are territorial. From this point on, let them together as often as you can, but keep them apart during the day or while you are gone, for another 3-4 days. Eventually the older cat will care for the young one as if it was his baby. It's a beautiful process. Get your camera out and save the memories!