When you turn on the computer, boot from your Debian USB. On the installer menu, select Advanced Options and then ... Expert install so that we can have more control over the installation process, as the defaults assume that you are setting up a generic desktop computer.
Select Choose language followed by your language and country/territory/area and locale. For the United States: you would most likely do English, United States, and en_US. Do not select additional locales, as they will not be necessary here.
Select Configure the keyboard and use whichever language applies to your keyboard type. In the United States, this would most likely pick American English.
Select Detect and mount installation media followed by Load installer components from installation media. Do not scheck nay boxes, as the required components will automatically load. What is listed there are optional components.
Select Detect network hardware and then Configure the network once that finishes. Press Enter to begin link detection with the default time, unless you need to change it for a good reason, and select your computer's Ethernet interface. Auto-configure the network. Once again select the default link detection time unless you need to change it for a good reason. Now enter your hostname. It would be good to name your digital signage computer based on where it is. For example, the Department of War could use ds-lobby.in.war.gov (or my personal suggestion for domain names: ds-lobby.in.war.fed.us) for a sign in the main lobby.
Select Set up users and passwords and allow logging in as root. Set a secure-enough password. On most days, however, you will never actually need to log on to these signs to do anything, as updates for both apt packages and Flan can (and should) be automated via cron jobs.
Select Configure the clock and use NTP with the server set to your organization's time servers, such as pool.ntp.aperture.akron.oh.us, along with your local timezone. In Akron, this would be Eastern.
Select Detect disks and then Partition disks. Select Guided - use entire disk, use the internal hard disk (or solid-state drive), put all files into one partition, and then write the partitions to the disk with their default sizes. When asked to confirm, do so.
Select Install the base system and wait. The amount of time this will take depends on the speed of your computer, USB device, etc. When asked about a kernel, select linux-image-amd64 with only targeted drivers/firmware.
Select Configure the package manager. Use a network mirror with the network protocol of your choice. Select to enter the mirror information manually, and use your trusted mirror option unless you feel strongly enough that the default choice won't have issues for you. For example, with ftp or http, you may use ftp.aperture.akron.oh.us as your mirror. Regardless of which mirror you use: keep the archive mirror directory as /debian/, and leave HTTP proxy information blank unless you absolutely need a proxy. Select no for non-free firmware unless you absolutely need it for odd components. Regardless of whether your computer has non-standard components requiring non-free firmware though, select no for non-free software, and contrib software, along with source repositories, as none of those are necessary here.
Select Select and install software, and choose security updates. Leave release updates and backports disabled. Since this will be a mainly unattended computer, have it automatically install security updates. Participate in the package usage survey, or don't, it makes no difference really. Select only standard system utilities, as everything else (including SSH) is unnecessary, and wait for the packages to install.
Select Install the GRUB boot loader. If asked for either: force installation to the EFI removable media path and update NVRAM variables to automatically boot into Debian. Do not have os-prober detect other operating systems, as this computer should not have other systems installed on it.
Now Finish the installation, select yes to have the system clock set to UTC, and then select continue to reboot. Eject the USB drive once you get back to the UEFI logo screen, and boot into Debian GNU/Linux. Once you get to the login prompt, sign in with your root username and password, and continue on to the process for installing Flan.
If your organization maintains its own local copy of Flan with customizations not present in the upstream version, replace the www.aperture.akron.oh.us URL with your local repo. For example, the University of Akron Information Technology Service's Computer Center may use something like http://www.cvs.cc.uakron.edu/flan/ for their version of Flan. In that case, their tarball link would be http://www.cvs.cc.uakron.edu/flan/flan.tgz?tarball=1.
If you would like a customized install, you will want to write a script (Perl, Bash, etc) to manage deployment. As an example of that, see the setup.bash script for use at the University of Akron's College of Business which sets a custom config.h and ico.xpm file to customize their signs with UA branding.