Using these steps you can install Redmine on Debian 8 using MySQL as database backend, nginx as web server and Unicorn as application server. Redmine will be installed at /srv/redmine/ path.
Debian 8 is the first version of Debian that is using systemd, all other steps are the same also for older versions of Debian.
Add user account that will be used for redmine application.
As a database will be used MySQL server.
Now make sure that redmine user can access Redmine files.
And following commands should be executed as redmine user.
To access newly installed instance open web browser and access it using hostnamed configured in nginx.