Personally I use Pendrive Linux YUMI
which allows you to set up a bootable flash drive with as many .ISOs as you can fit on it. To put Windows 8 on the flash drive you will need at a minimum 4GB flash drive (bigger is only necessary if you want there to be more .ISOs). What you will need to do is download YUMI and run the application, agree to the terms (just hit I Agree), find the drive letter of the flash drive you are planning on making into the bootable USB, then find Windows Vista/7/8 Installer (only one of them can be on the bootable USB at a time (it's a glitch they are working on fixing)), then browse to the place where you have your .ISO stored, select it and click open. Then click create and wait the time it takes to make it. Once that is done, click finish, and plug it into whatever you are putting the OS on and set it to boot from that flash drive. The YUMI interface is easy to navigate, and you just select Windows Vista/7/8 and then it boots to the install screen of Windows. After that just follow the regular Windows installation steps.