I am assuming that you mean post installation. This was discussed a short while
ago if you search back. It is quite simple to do, but you need to work from a
Live CD on unmounted partitions to resize your old installation to create space
for a separate home. Alternatively you could add a second drive. They are cheap
now or maybe you have an old one kicking around.

Here is a link: https://help.ubuntu.com/community/Partitioning/Home/Moving

You can do the repartitioning from the GUI using Partition Editor (System,
Administration) from the Live CD. The steps are:
1. resize (smaller) your Linux installation.
2. create a new drive in the freed space using the format of your choice, likely
ext3

You can copy home using Nautilus or follow the CLI instructions in the link.
Make sure that you get the hidden folders if you use Nautilus. The only step
that you can't do from GUI is the last one. However, I have done this using
System, Administration, Users and Groups by creating a new user and then using
the new home for it. Later you can delete the old use after making sure that you
can login successfully in the new account.