Creating a bootable USB installer

Hi!

I followed the guide () for creating a bootable USB drive. Unfortunately, the guide does not work for me.

Background: I have a 2008 iMac with the "question mark issue" and wanted to boot the computer from a USB drive. The OS installed was El Capitan, the highest OS possible for the computer.

Following the guide, I can only install a .dmg file. Upon opening it, the installer tells me there is a problem that made installation impossible.

Using the commands in terminal gets me the terminal message "command not found". This seems obvious to me, because the commands are not looking for a disk image....

Thanks for help!

Izi