IF… when you boot up holding down the option key, your USB stick does not
show up as a bootable source - then you may have to retry making the USB
boot stick. Be patient, give it time to possibly find your stick for booting.
IF… it does show up as a bootable drive when booting with the option key
down / but won’t fully boot from the stick when chosen as the boot device,
then again… try, try again to re-make a bootable USB stick.
Just did this recently using a USB 2.0 stick on an 800 MHz G3 iBook
and newer USB sticks would not work. And I was quite surprised that it
worked at all. And no, I did not need to use the laptop-specific install image.