Before downloading, ensure your Mac is compatible with El Capitan. Generally, most Macs from can run this version. Requirement Minimum Specification OS Version OS X 10.6.8 (Snow Leopard) or later Memory (RAM) 2 GB (4 GB recommended) Storage Space 8.8 GB available (up to 18.5 GB for upgrades) Processor 64-bit Intel processor Compatible Models Include: iMac: Mid-2007 or newer MacBook: Late 2008 (Aluminum) or newer MacBook Pro: Mid/Late 2007 or newer MacBook Air: Late 2008 or newer Mac Mini: Early 2009 or newer 3. How to Use the DMG to Create a Bootable Installer
Run this in Terminal: sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
You will need a USB flash drive (at least 8GB) named "MyVolume".