The first step to creating a bootable CentOS USB disk is to insert the flash disk on which you'd like to install CentOS. Next, determine the mount point of the flash disk by running:
sudo fdisk -l
If you have partitions on the flash disk you'll see a whole bunch of mount points for each partition similar to /dev/sda1. Please note that you should use the full disk mount point e.g /dev/sda and not /dev/sda1 or similar.
Next you should download the CentOS 7 minimal install ISO from https://www.centos.org/download/ and then run the following command to install the ISO to the flash disk:
sudo dd if=CentOS-7-x86_64-Minimal-1611.iso of=/dev/sda
*This assumes that the flash disk mount point is /dev/sda
You might end up waiting quite some time for the command to complete, and there will be no feedback as to the progress of copying the information to the disk. It will eventually complete, and you can safely remove the flash disk. All is now ready for installation :)