lunabyte nicely directed me to this tutorial on installing WPMU on cPanel (thanks, Lunabyte!).
It worked without a hitch - almost :)
So I'll add this tip:
My cPanel would not let me delete the .htaccess file, which blocked the installer from proceeding. So I found a workaround by changing the permissions on .htaccess to 777.
FYI, my VPS server would not allow me to edit httpd.conf on cPanel, so I had to submit a support request for them to do it, using the guidance on the tutorial.
I have one last question, though, that will help all others who face the same problem I had with the .htaccess workaround. It's this:
Should I revert the permissions for .htaccess back from 777 to the original 644?
(If that's not clear, I can elaborate.)
Thanks for all your help, everyone.