Here is an easy way to create a bootable OS X USB with any USB drive at least 8GB or larger.
Open the App Store and type in "OS X El Capitan" in the App Store search bar. Or Click Here.
Click download even if you are running El Capitan already. You will be asked for your Apple ID and password. Enter the information and download the OS.
Once downloaded, the OS X installer image will be automatically saved to your Applications Folder.
You can now insert your USB drive and open up Disk Utility. You can find Disk Utility by using the spotlight in the top right corner of your Mac screen.
Once Disk Utility is open, select your USB Drive from the list on the left side of Disk Utility interface. Once selected click the Erase button, keep all generic options for the erase disk field as follows:
Format: OS X Extended (Journaled)
Scheme: GUID Partition Map
Click Erase. This will take less then 5 minutes.
THIS IS CRITICAL, MAKE SURE NO OTHER DRIVES NAMED "Untitled" ARE PLUGED INTO THE MAC. THE NEXT STEP WILL EARSE THE FIRST DRIVE NAMED "Untitled" AND INSTALL OS X EL CAPITAN INSTALLER OVER THE LOGIC PARTITION.
In the spotlight type in Terminal and press return. This will launch the terminal application.
Copy and paste this command into the terminal window:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
This process can take up to 30-90 minutes. If more then 90 minutes have elapsed, something went wrong and you must try again.
After running the command you will see this massage:
Erasing Disk: 0%… 10%… 20%… 30%…100%…
Copying installer files to disk…
Making disk bootable…
Copying boot files…
Leave the computer plugged into the wall while this process is running.
After this process is done from the Desktop you can right-click on the drive that should now be named "OS X" and rename it to "El Capitan Installer". This will make it easier to recognize the drive when attempting to boot from it.
After renaming the drive, you can turn off your computer. When powering up the Mac immediately after pressing the power button, press and hold down the Option key. This will bring you to the Boot Device Selection Interface. You can now select your OS X USB and boot into it to install OS X on any Mac.
For older OS X Lion and Mountain Lion USB creation, please view the video below: