Making the installer drive
1. Connect to your Mac a properly formatted 8GB (or larger) drive, and rename the drive Untitled. (The Terminal commands I provide here assume that the drive is named “ElCapitan”. If the drive isn’t named Untitled, the procedure won’t work.)
2. Make sure the El Capitan installer (or at least a copy of it), called Install OS X El Capitan.app, is in its default location in your main Applications folder (/Applications).
3. Select the text of the following Terminal command and copy it:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/ElCapitan --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
4. Launch Terminal (in /Applications/Utilities).