

You'll be presented with the OS X Startup Manager, listing all bootable devices. When your Mac's screen turns off, hold down the Option key while your Mac reboots. To use the bootable flash drive as an OS X Lion installer: Insert the USB flash drive into one of your Mac's USB ports.

Use the Restore Disk Image to Drive dialog box to browse for the DMG file with the installation files for Mac OS X Yosemite in this case, and click the OK button to. In the warning dialog box, click the Yes button. On the left pane, you’ll see all the Windows PC drives listed, right-click the USB drive that you’re intending to use to reinstall Apple’s OS X and select the Restore with Disk Image option. (Either get it from App Store or buy its installation USB drive) Create Bootable USB Drive Plug in the storage device or pen drive into the Mac machine and launch its Disk Utility by navigating as Applications Utilities.
