Plug in your macOS High Sierra bootable flash drive. Hold down the option or alt (⌥) key on the keyboard and power on the device. When you see the boot selection screen as shown, release the option key. Use either the keyboard's arrow keys or the mouse to select 'Install MacOS High Sierra.' From the App Store, grab the High Sierra installer. It will launch 'Install macOS High Sierra' after the download completes. Click 'Continue', accept the licensing agreement, select the install drive, then the installer will prompt for an admin password. After that, it will begin downloading to '/macOS Install Data' on the local machine.