If it's an upgrade, then the system should recognise the /home partition
automatically.

If it's a clean install from CD, then when you get to the partitioning
stage, do a manual partition. Edit the partition on which you have your
/home and you'll get a dialog including a drop down list of where to
mount the partition - one of the options given is /home - also, make
sure you don't ask for the partition to be formatted.