Firstly don't panic just yet.. 24 hours is no time at all in terms of two cats getting to know each other. It can take weeks for a cat-cat relationship to settle down, when I got my second kitten it was nearly two months before peace broke out completely and now they are practically inseparable!
For now if you can try and keep the new kitten in one room with all the necessities (food, water, litter tray, somewhere to "hide) and keep the established cat out. You probably want to do this for 24-48 hours to give the new kitten some time to settle down where she won't have to worry about Mitch.
Once you are happy that the new kitten is eating, drinking and toileting bring some items into "her" room that smell of Mitch (blanket, toy etc) and give her chance to get a safe sniff of him. If you can, try and do the reverse as well.
Then you can try letting them "meet" each other again, make sure each has their own "safe space" to retreat to if they want. Keep food and water plentiful and ensure each has access to a litter tray and you'll want to be fairly prompt on keeping the litter clear as well until they get settled with each other as I've known some cats to refuse to use a tray if it contained the..erm..output of a "strange" cat.
