As posted elsewhere here. A site search would have pulled it up. A google search would have probably done the same.
Well, I'm primarily going through this to see if I've got it right myself (as I'm sure someone else will correct me if I'm wrong at any point), but really - you should at least make the pretense of having put some effort in yourself...
This assumes a RedHat system, if you're using something else, this will at least get you started.
1. Check that quotas have been set up in the default kernel on your system.
# grep CONFIG_QUOTA /boot/config-`uname -r`
Check that the return is "CONFIG_QUOTA=y"
2. Check that the quota package is installed.
# rpm -qi quota
3. Assuming yes, add quotas to appropriate areas in /etc/fstab - I'll assume /home on the second drive using ext3.
/dev/sdb1 /home ext3 exec,dev,suid,rw,grpquota 1 2
4. Create quota files (done automatically these days, I think) by remounting the selected filesystem and activating quotas.
# mount -o remount /home
# quotacheck -cgm /home
c - performs a new scan.
g - scans for group quotas.
m - remounts the scanned filesystem.
Once the command is run, you should be able to find the "aquota.group" file in the configured directory.
5. Run the "edquota" command to specify quotas for a particular group.
# edquota -g <group>
Differences between soft and hard limits can be found out from Google
6. Save the file.
8. Add a "quotacheck" command to your /etc/crontab to check for quota limits periodically.
...I think that's it - if I'm wrong, I'm sure somebody will post corrections.