I can only speak for a linux server install
I set the ownership (chown) to the web servers user, eg...
chown -R www-data wpmu
eg. the command line screen looks like this...
strixy@fnpos:/var/www$ sudo chown -R www-data wpmu
and set the permissions
chmod -R 755 wpmu
where
wpmu = your install directory (top)
eg. /var/www/wpmu
yoru server root being www
and you are not intending to run this from your document root(eg. domain.com/), but rather a testing page (domain.com/wpmu)
and where
www-data = your webservers username (it's www-data on Debian, Ubuntu, etc. It's "apache" - I believe - on RedHat and "www" on Slack. I know I'm probably wrong on the last one. 75% sure on the Red Hat one)
and assuming you have either used "su root" for root access or are prefacing the above commands with sudo
eg. sudo chmod -R 755 wpmu (debain, etc) from /var/www
eg. the command line screen looks like this...
strixy@fnpos:/var/www$ sudo chmod -R 755 wpmu
The '-R' is recursive, to include all files and folders beneath the target. (must be capitolized).
the 755 sets it to read/write for the owner and read only for the group/other.
You will have to adapt to your specific environment and needs, of course. YMMV
Helpful?