Install Mac OS X from a USB drive on a PowerPC-based Mac

When nothing else worked when trying to install Mac OS X Leopard onto my PowerBook G4 1.67GHz without the original installation DVD, here's what eventually did.

What you'll need

• PowerPC-based Mac you want to install OS X on by using a USB drive
• ...USB drive (16GB or more in capacity; 8GB might work, but come on, it's 2020)
• Second old Mac that already boots Mac OS X Leopard (other versions may work, but are untested. All I know is that my macOS Catalina hackintosh did NOT work)

Part 1: Preparing the disk image for restore

1. Boot up your second old Mac running Mac OS X Leopard
   
2. Download #31 (Leopard_10_5_4.dmg_.zip) from here and extract the DMG
3. Scan the DMG for restore using Disk Utility ("Images" -> "Scan Image For Restore...")
4. Lock the DMG (right-click -> "Get Info" -> check "Locked")

Part 2: Restoring the disk image to the USB

1. Format your USB as "Mac OS Extended (Journaled)" using the "Apple Partition Map" using Disk Utility
   
2. Restore the DMG to your USB using Disk Utility, making sure you check "Erase destination" (open the Restore tab, drag and drop the DMG to the "Source" field, drop and drop the partition you formatted in Part2Step1 to the "Destination" field)
3. Eject your USB drive and plug it into the PowerPC-based Mac you want to install OS X on
   

Part 3: Installing Mac OS X (okay kids, this is where it gets complicated - mostly copy/pasted from here; follow that if this doesn't work for some reason)

1.  Boot your PowerPC-based Mac into OpenFirmware (power it down and hold Command + Option + O + F while powering it back up)
2. Run dev / ls and look for the entry with "/disk@1" at the end of it (in my case "usb@1b,1")
3. Run devalias and look for the entry with the text you found in Part3Step2 at the end of it (in my case "usb0")
4. Run dir text_from_Part3Step3/disk@1:3,\System\Library\CoreServices (a.k.a. dir usb0/disk@1:3...) and make sure the entry that contains "BootX" also contains "tbxi" (if it doesn't, panic!)
   
5. Run boot text_from_Part3Step3/disk@1:3,\System\Library\CoreServices\BootX (a.k.a. boot usb0/disk@1:3...)
   
6. You should see a little spinner, then some white-on-grey text, and finally the Apple logo. Congratulations, you've successfully booted your accursed PowerPC Mac from a USB drive! The installation process is the same from here on out as if you were booting from an official Mac OS X Install DVD.