If you need instructions on how to make a USB flash drive for this tutorial, one for creating it on Windows can be found here. Creating the USB drive on a Mac running OS X is as simple as restoring an image with Disk Utility. The Leopard disc image used in this tutorial can be found here.

Note: This tutorial assumes you have already created a bootable USB drive containing Leopard.

1. Shut down your PowerPC Mac.

2. Plug in your Leopard USB drive into your Mac.

3. Turn on the Mac. Just as you hear the boot chime, hold down Cmd/Command/Apple key, Option/Alt, O, and F simultaneously. Continue holding them until you see the Open Firmware screen. It should look like this:

4. In this window, type dev / ls. You will see a list scrolling down your screen. As it does so, make note of anything labeled such as "/usb@1b" with a "/disk@1" or similar. Once you've located your "/usb@x" that is paired with a "/disk@x", you will want to look at what is above it to find something labeled "pci@XXXXXX" or similar. The screen you are seeing is outlined in a tree format, with many branches leading to your flash drive. The first branch in your tree is this "pci@XXXXXX" directory, which then leads your Mac to the USB port, then your flash drive. I have highlighted below what to look for:

5. Now, let's organize these elements so that we can form the command we need to boot your USB drive. In my example, my drive is located at /pci:f2000000/usb@1b/disk@1.

6. We'll need to create an alias to boot from, so your Mac knows where the USB files are in language it understands. Depending on your Mac, your command will be different. The command is: devalias ud /pci@f2000000/usb@1b/disk@1. Fill in everything after "devalias ud" with the information that matches your Mac. If you need to, you can run the dev / ls command again. Once you've entered the command, press Return.

7. To verify you have done everything correctly so far, type dir ud:3, and press Return. The "3" used here is the usb's boot partition identifier. This may differ if you use a different Leopard image. If you've done everything correctly, the screen will show a listing of what is on your USB flash drive, and should look this:

8. Now, we are ready to boot into the Leopard installer. Type boot ud:3,\System\Library\CoreServices\BootX

9. With any luck, the next thing you will see is the Apple logo. It may take a while for the installer to load, as most PowerPC Macs did not have very fast USB ports.

10. Eventually, the Leopard installer should load. You can then proceed with installation as needed!

This tutorial is derived from this page on MacWorld.