If the partition exists, then it need formatting. Once that is done, you will need an entry in /etc/fstab.
Here is what it looks like for my /home ...
/dev/sda6 /home ext3 defaults,noatime 1 2
Things that may differ, the first field, /dev/sda6 will depend on the hard drive type. This is a sata drive, hence the sda. The 6 is the first user partition on an extended partition.
ext3 is the file system type. This will change depending on the file system type you select when formatting. The rest should get you going.
Once you edit fstab, as root, then a command 'mount /dev/sda6' or a 'mount -a' shoudl mount the partition. No need to boot.
BTW, 30g for a / is overkill. It won't hurt, but you are very unlikely to fill that up. User files should never go to your / partition. Just the system and if you like, applications.