Installation instructions:
1. Copy checkquota script to your server. You can do this by creating the file on your server and copy/paste the source code. Or, ftp the file. If you are ftp'ing the file, make sure you do it in ASCII.
2. As root, save the file as /usr/local/sbin/checkquota and set permissions to 755
3. From an ssh command line, type 'rehash'
4. From an ssh command line, run checkquota -h to view available options

You can find a copy of the source code here

Here is a sample of how this was installed on one of my servers, while logged in as root:
vi /usr/local/sbin/checkquota (I then copy/pasted the code from here into the vi session and exited
chmod 755 /usr/local/sbin/checkquota
rehash
checkquota
checkquota -h

If you wanted to create a cronjob that runs every day checking for overquota users, add an entry like this to your root crontab (this would run daily at 1am):
#checkquota
0 1 * * * /usr/local/sbin/checkquota