Any suggestions on introducing a new kitten to an older cat who's not

[loli_cakes]

handling it well? I know this is a fairly common question when it comes to us cat noobs, but I thought I'd get some suggestions. We just brought a kitten in yesterday who's about 3 months old and instantly we let him roam (suggestion of the lady who worked at the shelter) well, my older female cat of four years attacked him. He's been put up in another room which he hates and I'm sort of at a loss of what to do. She is obviously not happy about the newcomer and I was just wondering how normal this is? The aggressive behavior that is, beyond just hissing. I was also wondering how long is usually takes for two cats to finally accept each other, or how long it took for yours. Thanks!