{Q}How to flash firmware via ODIN? topic

I was trying to root my sisters S3 i9300, until the Auto Rooting App i used, didn't work and changed the kernel(without root access available :mad:) and now whenever i try other rooting apps it doesn't work. I think its because of the kernel change it wont let me root. So someone told me to "Get back a stable system by flashing latest firmware by Odin. If it boots normally, use cf-auto-root (by Odin, too). ".

I got ODIN 3.10.6.
Problem:How to flash latest stock S3 i9300 firmware(link would be appreciated) by ODIN?
How to then use CF Auto Root, to root my device?
A video or clearly detailed written tut would be awesome :)

I know is quite noobish question;) but YOLO?