Creating an OS X El Capitan install flash drive

Format the USB Flash Drive using Disk Utils:

http://www.macworld.com/article/2990837/storage/how-to-format-a-startup-drive-in-el-capitan.html

The volume name in the next step is /Volumes/name-you-gave-the-volume-in-the-first-step.

Copy install files to the Flash Drive using createinstallmedia:

http://www.macworld.com/article/2981585/operating-systems/how-to-make-a-bootable-os-x-10-11-el-capitan-installer-drive.html

To boot from the flash drive, reboot your Mac holding down the Option key and the choose the icon for the flash drive.