To boot Arch Linux from a USB, you need to create a bootable USB drive with the Arch Linux ISO image. For this purpose, download the ISO file of Arch Linux and make it bootable using Etcher or Rufus. Once you have created the bootable USB drive, insert it into the computer you want to install Arch Linux on. Restart the computer and enter the BIOS settings. Change the boot order to boot from the USB drive, save the changes, and exit the BIOS settings.