Not sure where you're getting tangled here or how much you know about OS X Server 10.6, so...
AFAIK, Snow Leopard Server wasm't available as a download, it was a separate DVD distribution.
Unlike more recent versions of OS X Server, the 10.6 server distributions were usually standalone DVD disks, as well.
(There is a slight wrinkle if your Mac needs a newer version of 10.6 than your server distribution; there was a different streaming upgrade process for that. IIRC, the appropriate OS X client version was installed first, then you booted that client version and loaded the DVD and installed OS X Server.)
But if you have a Snow Leopard Server distribution that supports your particular Mac Mini model, then boot the DVD, use Disk Utility from the Utilities menu in the bootable DVD environment, wipe the disk, and then install OS X Server.
You'll need a 10.6 license key, if you don't already have that.
FWIW, here are the available manuals.