Please download from the App Store DiskMaker X
- Download the OS installer and DiskMaker X, if you haven’t already.
- Insert an 8GB flash drive.
- Start up DiskMaker X, choose Yosemite from the list of options, and it’ll handle the rest for you.
When its done, you can insert that drive into any Mac and launch the installer by holding Option when you boot the computer. Besides installing Yosemite, this drive will have a couple handy utilities on it, like Disk Utility and Time Machine recovery.