I've never used the guided option but always manual. You should see a 'prepare partitions' window where you can select free space by mouse-clicking to highlight it. Then click the Add tab below the main window and you should get a new window 'Edit partition' where you can select the partition size, and under Use as the filesystem type (ext4 is default). Check the box to format the new partition and click the down arrow to the far right of Mount point. You should see a number of options and the first should be the forward slash (/), the symbol for root. Make sure that is showing in the Mount point box and click Ok.
You will see all of this with the manual option, I don't know if you will with the 'guided' option.