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Follow the instructions at http://otrs.github.io/doc/manual/admin/stable/en/html/index.html, do a standard install and make sure everything works.
Pay special attention to the phrase "Please install OTRS from source, and do not use the OTRS packages that Debian/Ubuntu provides." 
It will require some fiddling to get all the Perl modules sorted, I suggest to use the packages modules as much as possible.
The docs all seem to assume that you'd want to run OTRS inside a subdirectory (https://example.com/otrs), but we want it to be the root of our vhost (https://otrs.example.com), in which case this configuration is a little bit different, see below (you should have the HTTPS stuff already configured, probably in /etc/apache2/mods-enabled/ssl.conf):
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