If you are going to boot from a USB you can choose to do so from BIOS. If you want to install a full OS onto the USB I'd recommend installing grub and installing the OS on the USB. If you do so to get back into your base OS type these commands into the grub console.
set root = (hd0,1)
Never do this if you don't know what you are doing because you could end up stuck at grub rescue which makes it hard or even impossible to recover from.