So the lower-ish difficulty answer would be to run the iso installer in a VM with the usb stick forwarded to that VM.

Or you can learn what those fancy installers do: on debian you would use debootstrap

Here seems the whole guide on how to install debian manually with it:
https://gist.github.com/tr3buchet/6407920

Btw, this is also basically how you install Arch. As of until recently there wasn’t any installer and you had to go through each step manually (create partitions and fs, install the base system with <insert distro specific tool>, chroot, update fstab, distro specific finishing touches, voilà)