Confusion about creating a bootable flashdrive
You may want to delete your answer to your own question. 

In fact:


Insert bootable flash drive 
Open said drive and launch the Install Mac OS High Sierra app
Follow the prompts


You could also reboot the system with the flash drive inserted and holding the Option key. When the drive selector comes up choose your flash drive and installation will commence.

If you run the OS install app from the Applications folder it will install it onto your boot drive. You could do that to install the OS on a thumb drive (just select that drive during OS install) as that makes a good emergency boot drive but it won't make an installer drive.