Basically all you need do is follow the directions already found here in Stephens howto. . There is a caveat for /usr though. After copying /usr to another partition you must change the permissions of /tmp to 1777 or you will get .xsession errors.

chmod 1777 /tmp

Here are my partitions, their sizes (in MB) and respective mount points:

Filesystem Size Used Avail Use% Mounted on

/dev/hda3 2.8G 1.1G 1.6G 40% /
/dev/hda6 942M 184M 711M 21% /home
/dev/hda7 942M 673M 222M 76% /tmp
/dev/hda8 942M 589M 306M 66% /var
/dev/hda14 5.5G 3.1G 2.2G 60% /usr

Notice that /usr is fairly large.