Short version:
1. Install OS4. Leave space on the drive for Linux partitions.
2. Create Linux partitions using Media Toolbox. Use the filesystem identifiers from the above install guide. The swap partition should be twice the size of your installed RAM.
3. Boot the Debian CD from UBoot. I think the command is diskboot 500000 x:0 0
where x is the unit number of your CD drive (0-3)
4. Pick up from step 3 of the Debian part of the install guide. Obviously you'll want to use the current kernel version numbers and your Linux partition number instead of /dev/hda4
It's been literally years since I installed Linux on my A1, so things may have changed more now that the Etch installer has appeared.