Open the Boot Camp Assistant app in / Applications / Utilities / (or launch it with Spotlight).Connect the USB flash drive to the Mac – this will erase and turn into the bootable Windows installer.Have the Windows ISO file downloaded and found somewhere easily.
While we’re focusing on creating the installer for now, you may want to make a backupup start and finish up of the Mac with Time Machine, especially if you are not familiar with formatting drives. How to Create a Windows 10 Installer Drive from Mac OS X with Boot Camp Assistant