Booting USB is really easy! I have done it twice in the past 2 days on my DLSD PowerBook G4
1. Enter Open Firmware (CMD+OPTION+O+F)
2. Type "boot ud:,\\:tbxi" (No quotes) and hit enter
3. You should get the Apple logo with the spinning wheel if you a. Restored the image properly or b. if you typed the OF command correctly.
To restore, obtain a OS X Leopard image, the Macintosh Garden is a good place to start, they have loads of PPC software. Once you have your image, follow the instructions below. (You may need another Mac, as Linux may not work the same)
1. Get a USB that is 8GB or greater in size.
2. Open Disk utility
3. Click on your USB in the side bar and navigate to the partition tab, choose 1 partition, click on options and make sure "Apple Partition Map" is used. Then Apply
4. Click on the restore tab, choose the image as the source, your USB partition as the destination and youre done!