Bootable usb

You can download the Mountain Lion installer here ... but I have never seen any instructions on how to make a bootable USB installer drive out of it except for this old article from Macworld in 2012: https://www.macworld.com/article/218583/how-to-make-a-bootable-mountain-lion-install-drive.html

AFAIR I have used the MacWorld method MartinR mentioned.

But AFAIR I have also used this simpler cookbook that basically does the same thing:

Copy the following file out: Install OS X Mountain Lion.app > Contents > Shared Support > InstallESD.dmg.

Open Disk Utility and select the Flash Drive device name (not its volume or partition). Select the Erase tab on the right and then set format to Mac OS Extended (Journaled) and GUID Partition Map.

Select the Flash Drive volume (not the device), select the Restore tab at the top and then select the InstallESD.dmg. Click Restore and wait for the restoration to finish.