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
This version has an extra Drive menu option. It allows you to set any one of the four partition table entries in the MBR as the first partition or reset the order of all 4 entries so that they are are in ascending positional order (the same order set by diskpart or Windows). This means you can have up to 4 partitions on a USB Flash drive (e.g. created with EaSeus Home Partition Manager) and then quickly make any one of the partitions visible in Windows so you can change the files on them using Windows Explorer.
This feature is also available in RMPartUSB, so that you can use RMPartUSB in a script to load up to four partitions with files (e.g. using XCOPY). The new parameter is FIRSTPART
RMPARTUSB DRIVE=2 FIRSTPART=3 will set the 3rd entry in the partition table to the 1st partition and will swap the 1st entry to the 3rd position.
I got an email asking me to figure out how to put a correctly licensed version of O&O DiskImage Professional 6 onto a multiboot USB stick. It turns out the USB drive volume label needs to be set to OODIPRO in order to get it to work. See the full tutorial here.
This tutorial shows how to download and extract bcdboot, imagex, oscdimg, etc. from the WAIK in about 30 seconds instead of having to download the entire WAIK ISo and then install it. A very useful tools by JFX.