With the device powered off, disconnect any external hard drives and flash drives.
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." Press [enter] or click your selection with the mouse.
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