If you are just dipping your toe into the water and want to try USB booting, I suggest you first try Easy2Boot.
Easy2Boot is based on grub4dos and allows you to boot pretty much anything from a USB drive.
Once you have a working Easy2Boot drive, you can look at the grub4dos or other tutorials on this site.
The tutorials are numbered in chronological order (Tutorial 1 was the first one I wrote and Tutorial 120 is the 120th one written a few years later), therefore the higher numbered tutorials will generally contain better and more current solutions.
For testing USB drive booting in a Virtual Machine, check out Tutorial 4