How to Boot a MacBook from a USB Drive

First you need to install a bootable operating system on a USB flash drive. If you have a new MacBook, then most likely it does not have a USB input - only USB-C. In this case, you need to buy an adapter from USB to USB-C. After inserting the bootable USB flash drive into your Mac, turn on your Mac and immediately press and hold the Option button until you see the boot menu with a drive selection.