Step 1: Download The ISO Image
To get a copy of CentOS 7 download from its source mirror. CentOS 7 is now shipping for 64 bit platforms, and currently there is no 32 bit ISO image. This is primarily due to the fact that most servers in production are 64 bit.
Step 2: Make A bootable Driv
After you have downloaded the ISO image, make a bootable USB drive. Since Centos 6.5, you can create usb drive bootable by simply transferring iso file to usb using dd command. Win32DiskImager is an alternative solution as we found Unetbootin no longer works for CentOS 7.
# dd if=/iso/CentOS-7-x86_64-DVD-1602-99.iso of=/dev/sdb