Installing Debian over serial
Installing Debian using serial is relatively straight forward and still very useful on a multitude of devices that can be installed, and managed, over serial. This include the PC-Engines APU Devices which can be booted on USB and installation performed over serial.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.”-
Prerequisites (the beginning)
Serial cable for your device.
Serial terminal, such as minicom, picocom, GTKTerm, screen or Putty  if you’re on Windows.
Debian 11, code name bullseye, netinst, for 64-bit PC (amd64)  or a media with additional non-free firmware, such as .
USB Device to transfer the Debian ISO to.
Note: Debian netinst works well for APU devices as they're using AMD CPUs and Intel NICs - both vendors have been good Open Source contributors.
Burn ISO, burn to USB
As we will boot the system from USB, we will have to write the ISO to a suitable USB drive.
On Linux you have the option of using dd:
Replace sdn with sdb, sdc, depending on what drives are already installed in your
system. Please double-check that you’re not writing to other drives
than the USB for this purpose. You will lose data if you’re not
careful. (ask me how I know).
There’s also the graphical tool called popsicle on many Linux distros which is quite neat.
On Windows, you can use Rufus 
Note: The netinst iso is around 400M, while the "firmware" iso is just shy of 4G.
Now plug the USB and the serial cable into the system, start your favorite serial terminal and power up the system to be installed.
Hopefully you will now see Debian booting and, after a short while, show the installation menu.
BUT… if you just choose install now, it won't work, as the console is not shoved over serial for the rest of the installation, unless you do this.
<TAB> key instead and replace quiet after the
Note: Adjust port and its parameters to your specific device. The above works for APU boards (ttyS0 is the first serial port on that - please note uppercase S followed by a zero)
Then when the message below shows up, press space to start
installation, or wait 30 seconds.
You should now be able to complete the standard Debian installation process. Do remember to remove the desktop options and add SSH Server.
You should now be able to connect over SSH to the system.
GRUB_SERIAL_COMMAND="serial --speed=15200 --unit=0 --word=8 --parity=no --stop=1"
Note: I have not had this problem with the latest builds, and have not taken the time to reproduce if it was my mistake or certain builds that caused it.
 Debian 11 netinst: https://www.debian.org/distrib/netinst
 Unofficial Debian 11 with non-free firmware: https://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/11.3.0+nonfree/amd64/iso-dvd/
 Putty for Windows: https://www.chiark.greenend.org.uk/~sgtatham/putty/
 Rufus: https://rufus.ie/en/