Installation with Keyboard/Monitor
That is only supported on Indy's(?) and Indigo2 systems with XL graphics.
Power on the machine and click "Stop for maintenance". Then select "Enter Command Monitor".

Installation via Serial Console
Connect the serial console and boot the machine. Go into the PROM menu and select "Enter Command Monitor".

Preparations
Unset a previously set netaddress: "unsetenv netaddr". Maybe you want some hardware information: "hinv".
You need a DHCP, bootp and tftp server in your network (google for some setups, maybe i will add a howto about that later). Add an entry to the DHCP server config with the mac-address of your box (see above). Give it an IP, a hostname and the path to a tftp image.
Back to the SGI box: now type in "boot -f bootp()$tftpimage $options" (there are some more ways of typing that command, i used this one because it worked). Replace $tftpimage with the name of the approtiate image for your box (even though the dhcp server provides that name, it is sometimes ignored). See the documentation of your Linux distro for $options.

Installation
Partition your HardDisk - if needed (blank disk), create an SGI disklabel with "fdisk": x -> g -> w. Restart fdisk so that the changes take effect.
Now you can partition the disk as usual and follow the installer to initialize the partitions (with the following order: swap, /, rest).
Configure the network, install kernel drivers and modules (via network), configure modules (maybe add sg and sr_mod for external scsi cdrom), install the base system and do everything else you like.

Make System bootable
Make it so. Reboot the system, enter the prom and set it up for booting your install:

        setenv OSLoader linux
        setenv SystemPartition scsi(0)disk($X)rdisk(0)partition(8)
        setenv OSLoadPartition /dev/sd$Y

where $X is the scsi id of your disk and $Y is the linux device name of your root partition (e.g. b1 -> /dev/sdb1, a3 -> /dev/sda3).
From now on you can safely boot your Linux install like an IRIX install with the "start system software" option. Continue to configure your install like every other debian system.   Have Fun!