Select the text of this Terminal command and copy it:
sudo /Applications/Install\ OS\ X\ Yosemite\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite\ Beta.app --nointeraction
Launch Terminal (in /Applications/Utilities).
Warning: This step will erase the destination drive or partition, so make sure that it doesn't contain any valuable data. Paste the copied command into Terminal and press Return.
Type your admin-level account password when prompted, and then press Return.
The Terminal window displays the progress of the process, in a very Terminal sort of way, by displaying a textual representation of a progress bar: Erasing Disk: 0%... 10%...20%... and so on. The program then tells you it's copying the installer files, making the disk bootable, and copying boot files. Wait until you see the text Copy Complete. Done. (see the screenshot above), which could take as long as 20 or 30 minutes, depending on how fast your Mac can copy data to your destination drive.
You now have a bootable Yosemite-beta install drive. If you like, you can rename the drive from its default name of Install OS X Yosemite Beta, though I think it's kind of a catchy name.
Option 2: Use Disk Utility
You'll find Disk Utility, a handy app that ships with OS X, in /Applications/Utilities. Here are the steps for using it to create your installer drive. The procedure is a bit more involved with Yosemite than it was for Mavericks (which was itself a bit more involved than under Mountain Lion and Lion).
Once you've downloaded the Yosemite beta, find the installer on your Mac. It's called Install OS X Yosemite Beta.app and it should have been downloaded to your main Applications folder (/Applications).
Right-click (or Control+click) the installer, and choose Show Package Contents from the resulting contextual menu.
In the folder that appears, open Contents, then open Shared Support; you'll see a disk image file called InstallESD.dmg.
Double-click InstallESD.dmg in the Finder to mount its volume. That volume will appear in the Finder as OS X Install ESD; open it to view its contents.
Several of the files you'll need to work with are hidden in the Finder, and you need to make them visible. Open the Terminal app (in /Application/Utilities), then type (or copy and paste) the following command, and then press Return:
defaults write com.apple.finder AppleShowAllFiles 1 && killall Finder
(This tells the Finder to show hidden files--we'll re-hide such files later.)
Launch Disk Utility (in /Applications/Utilities) and then drag BaseSystem.dmg (in the OS X Install ESD volume) into Disk Utility's left-hand sidebar.
Select BaseSystem.dmg in Disk Utility's sidebar, and then click the Restore button in the main part of the window.
Drag the BaseSystem.dmg icon into the Source field on the right (if it isn't already there).
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