Posted 18 June 2012 - 09:05 AM
First, you need to create at least five partitions, one primary and four logical.
The first should be at least 200 MB in size and formatted as Ext2. This would be used as the /boot partition, and your dual boot setup will boot from here.
The 2nd and 3rd are root partitions for your OSes. You should choose the size; around 20 GB each is OK, but use more if you have lots of space.
The 4th and 5th are swap partitions for your OSes. Their sizes should be 1.5-2x the size of your physical RAM (important if you want to use hibernation).
If you want, you can set up a large 6th partition as a shared /home partition between the two OSes.
If you want to add more operating systems, add more root and swap partitions.
You should be able to choose the mountpoint (or "role") of the partition using the OS installer's partitioner (do not format the partitions!). The installer should do the rest for you.