This step-by-step procedure will teach you how to create a bootable USB drive installer for Mac OS X El Capitan. This method will also work for the other iterations of OS X (including the Yosemite, Mavericks, and Mountain Lion updates).
There are several prerequisites necessary for a successful installment of El Capitan. A list of requirements for the compatible models can be found below.
MacBook (released early 2015),
MacBook (aluminum series released late 2008 - early 2009 or later),
MacBook Pro (released mid/late 2007 or newer models),
MacBook Air (released late 2008 or newer models),
Mac Mini (released early 2009 or newer models),
Mac (released mid 2007 or newer models),
Mac Pro (released early 2008 or newer),
Xserve (released early 2009).
OS X 10.6.8 or later,
2 GB of RAM (minimum),
8 GB of storage space,
Some features require an Apple ID,
Some features require a compatible Internet service provider; extra charges may apply.
STEP 1: Download El Capitan
Launch the App Store app and sign in with your iCloud account. Click on OS X El Capitan > Download to begin the download process. The El Capitan download will require about 6 GB of storage space.
Once the installation package has been downloaded, it will launch automatically. Do not click on Continue.
You may quit the installation by clicking on the Install OS X menu > Quit Install OS X. Leave the installation package (Install OS X El Capitan) in the Applications folder and proceed to the next step.
STEP 2: Prepare the USB Drive
You will need a blank 8GB USB key. Connect the USB key to your Mac and launch the Disk Utility located in Applications > Utilities. Select your USB drive (not its existing partition, but the physical key itself). Click on Partition Layout and select 1 partition. Make sure that GUID Partition Table is selected and then click on Options > OK.
Select Mac OS Extended (Journaled) as the default format for the drive. Rename your USB key to something more significant (e.g. elcapitan) and make a note of it somewhere. You will need the exact name for a terminal command in STEP 3.
Click on Apply > OK.
STEP 3: Creating the Installation Key
Launch the Terminal application located in Applications > Utilities and type the below command:
Replace name_of_USB with the name you have chosen for your USB key.
Confirm with the Enter key, and type your administrator password when prompted. Let the process unfold until the Terminal window says Done.
Your OS X El Capitan installation USB key is now ready.
Updating from OS X Snow Leopard and Leopard
Additional steps are required when updating from Snow Leopard (Mac OS X 10.6.x).
Click on the Apple menu > Update software. Update to Mac OS X 10.6.8 before downloading El Capitan from the App Store.
El Capitan will install directly onto your Mac. You will need to download it once more in order to get the installation package (installer). Furthermore, if you have Leopard (Mac OS 10.5) and would like to upgrade to El Capitan, you must first purchase and install OS X Snow Leopard.
This method creates a USB key called Install OS X El Capitan. The key contains an installer (installation package) named Install OS X El Capitan. To perform an installation from the USB key, connect it to your Mac and boot your system while holding the Alt key down. Select the USB key from the boot options and follow the onscreen procedure to install the new operating system.
N.B. If at any point the installation procedure seems to stall, just let it be. Despite appearances, the installation procedure is likely just working slowly.