If you want to start your installation or boot a live system from USB, there are two great methods. Here, only one method will be explained. If your system does not support booting from a USB device, a method of enabling it to is described below. First, however, you have to prepare your USB device with UnetBootin.
First you have to tell your BIOS to boot from USB (if your BIOS supports it). When you switch on your PC, you usually get a message how to enter the BIOS setup. In most cases you have to press one of these keys: F2, F8, F10, Del or Esc
Then you have to select “Boot” and open “Boot sequence”. Now you have to bring USB or USB-device up to the position of the “First Boot Device”. Depending on your system, this dialog can look slightly different.
Older PCs often do not support booting from USB, therefore we need the Plop Boot Manager, which allows us to boot either from CD/DVD, USB, floppy or harddrive.
There is a very good guide how to use PBM on the authors website - so it will not be described here.