[4/24/15: Updated for Jessie stable]
Now that you have your newly burned and scorching hot install disc handy, put it in and boot your Mac holding down the "c" key. The first screen will have a few rudimentary instructions including how to choose your desktop environment (Gnome, KDE, LXDE, XFCE), but for an Openbox install I skipped that and just pressed return at the boot prompt. The next few screens are pretty self-explanatory, asking for language and keyboard settings. Just pay attention to the navigation instructions on the bottom showing how to use the tab, spacebar, and enter keys.
Next, you may see a screen that asks if you want to install nonfree firmware. I think this is due to first generation Airport cards needing nonfree firmware installed in order to work, and you need the Airport card to work if you're installing over a wireless connection. However, since I have an ethernet connection, I was able to answer no and move on. I'll set up wireless after I've installed the base system. For more information on this, check Debian's install manual and the section, "Loading Missing Firmware".
Now the installer will prompt for a hostname. This is what you want your computer to be called (I'll call mine "icebook"). Then it asks for a domain name, which I have no need for so I left it blank. Then it asks for a root password, your full name, username, and user password. Remember your passwords! Then after you choose your time zone, you're taken to the real meat of the installer--the Debian partitioner.
If you're installing a Debian-only system, you can use Guided Partitioning here. But if you're dual booting OS 9 or OS X, you'll want to select Manual Partitioning. When you next see the partition table, use the arrow keys to navigate down to your Debian partition (IMPORTANT: Don't change the small Apple partition(s) at the beginning of the table. OS X machines should have only one, but OS 9 machines can have eight.). If your Debian partition isn't already labeled "Free Space", press return and then choose "Delete the partition". When you return to the partition table, your Debian partition should say "Free Space". Now you will create the bootloader, swap, and root partitions.
Navigate down to Free Space, press return, and select "Create a new partition". Enter 1 MB for the size, to be created at the beginning of the partition. Then name it "Apple_Bootstrap". For "Use as:" select "NewWorld boot partition", then set the bootable flag to "On". Then select "Done setting up the partition" and you'll see the bootloader partition in your new partition table.
Next, create a swap partition following basically the same procedure. Go down to Free Space, create a new partition, enter the size (should be at least as much as your physical RAM for hibernate to work properly), at the beginning of the partition, but for "Use as:" select "swap area". Then select "Done setting up the partition" and see the new partition table.
Finally, begin the same procedure to make your root partition. Go down to Free Space, create new, and for size use the maximum remaining space (unless you're one of those weirdos who uses a separate partition for your home folder /wink). Name it something like "debian". For "Use as:" select "Ext4 journaling file system". For mount point, choose "/ - the root file system". For mount options, select "noatime" by ticking it with the spacebar. Finally you can select "Done setting up the partition" and see your completed partition table. Don't worry about the few kilobytes of free space at the end.
Now you can finally choose "Finish partitioning and write changes to disk." Use the tab key to select yes and press return.
This is when the actual installation begins. Here it will install the base system and, after a bit, ask you to enter your country and select an archive mirror. When it asked for proxy info, I left it blank since I'm not using one. The next question to come up is "Participate in package usage survey?" Answer yes because it's always good to let the mothership know there are PowerPC users out there.
On a subsequent page you'll be asked to choose additional software to install. Here you'll notice "Debian desktop environment" is checked by default. This will install whichever desktop environment you chose--which you could totally do. I'm not judging you, really. But if you want to install Openbox without all the extra cruft, uncheck this box by pressing the spacebar. You can always install XFCE later if you insist ;) Just make sure "Standard system utilities", "Print server", and "SSH server" are checked, then use the tab key to navigate to "Continue" and press return.
When the installation finally completes, the CD will eject and you can press return to boot into your new system.
Next part, "Installing the GUI". Hope to see you soon!
Part I - Pre-Installation
Part III - Installing the GUI
Part IV - Configuring Stuff
Part V - Bugs & Quirks