Let us begin with a disclaimer: Mac OS X 10.11 El Capitan is not fully cooked and therefore might do some funny things to your Mac. Proceed with caution with this prerelease software that could, in Apple's words, cause "crashes, freezes, kernel panics." I suggest you install El Capitan on a secondary Mac if you have one or, failing that, on a separate partition on your primary Mac.
I chose the latter since my backup Mac is ancient and didn't make the cut for El Capitan's minimum system requirements. Thankfully, creating a partition is easy and Brian Tong can show you how.
In a nutshell, launch Disk Utility, select the disk you'd like to partition from from the left panel and then click the Partition tab. Next, click the "+" button below the Partition Layout diagram. Choose a size for the new partition by dragging the divider (El Capitan is only 6GB) and give it a name. You can leave the Format as Mac OS Extended (Journaled). Lastly, click Apply and wait a minute or two for the new partition to be created.
With your backup Mac or partition ready, head to the Apple Beta Software Program site. You will need to sign up to take part in the program, if you haven't already. The process takes only a minute or two and requires your Apple ID.
Now, here is where we remind you again of the potential dangers of using beta software and the importance of backing up your Mac before installing El Capitan. With Time Machine, Apple couldn't make it any easier to create a backup. Go ahead, we'll wait right here.
With your Mac freshly backed up, click the blue download button from Apple's Beta Software Program site, which should open the Mac App Store and begin your download. That was not my experience, however. The blue download button didn't do anything for me, so I had to use the redemption code listed below the button manually in the Mac App Store to begin downloading El Capitan.
After El Capitan downloads, you will need to choose where to install it. If you created a partition, click Show All Disks from the installer window and select your partition before clicking Install. (Then to choose which partition to load on restart, hold down the Option key as your Mac boots up.)
Remember, the purpose of a beta is to iron out any kinks in the software. If and when you encounter an issue, report it to Apple using the Feedback Assistant, which you can find via its purple icon in the Dock.