There is a very simple way to make an OS X 10.10 Yosemite Developer Preview installer.
You need is the full installer of OS X 10.10 Developer Preview and a USB Drive (or something similar)
Since OS X 10.9 Mavericks, OS X now has a new executable file createinstallmedia to make a bootable usb.
Command to make an external Mavericks installer
This is assumes you have a an external partition named "Install OS X 10.9"
Make changes where appropriate.
sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/Install\ OS\ X\ 10.9 --applicationpath /Applications/Install\ OS\ X\ Mavericks.app/ --nointeraction
You will see the following:Erasing Disk: 0%... 10%... 20%...100%...
Copying installer files to disk...
Copy complete.
Making disk bootable...
Copying boot files...
Copy complete.
Done.
Command to make an external OS X 10.10 Yosemite installer
This is assumes you have a an external partition named "Install OS X 10.10"
Make changes where appropriate.
sudo /Applications/Install\ OS\ X\ Yosemite\ Developer\ Preview.app/Contents/Resources/createinstallmedia --volume /Volumes/Install\ OS\ X\ 10.10/ --applicationpath /Applications/Install\ OS\ X\ Yosemite\ Developer\ Preview.app/