I would like to install a plain Debian operating system on my pi without all the software and config that comes with the Raspbian image.
Is there a guide for this?
You will need some of the boot magic and a kernel that work on the board. These don't come with mainline Debian. You might be able to install these on a fresh card from some source (anyone have instructions?) or you might use a ready SD card image, but wipe the root partition to leave just the booting bits.
Then you might be able to use that kernel and a Debian install initrd (maybe netboot versatile?) to boot into the installer. I did this while installing debian on an NSA310 and it works if the kernel is adequate and you can manage the boot process. You might also look at the Raspbian installer kit.
Alternatively, you could use a second machine with a card reader and use debootstrap. If that ends up being cross install (e.g. amd64 installing for armel), you might need a few tricks. There should be several documents around describing debootstrap for arm boards. This I've only tried on i386, but will probably try for arm in the future.