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/