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.