Once the download has completed, insert your USB drive and you’re ready to go. If you’ve already downloaded the file–it’ll be listed as Open (otherwise, click Download). To get this software, open the App Store and search for “High Sierra.” After clicking on the entry, you will either see an Open or Download button. The first thing you must do is download the installer file from the App Store. I’ve used this inexpensive solution without fail. Don’t forget, if this is a standard USB flash drive you’ll need an adapter for the USB C ports on the laptop.

You don’t want to create your installer drive and accidentally erase the installer files while working with a spreadsheet or presentation. I highly recommend at least a 12GB flash drive that will be used solely for this purpose.

You will need an internet connect and a flash drive with a minimum of 5GB of space. I’ll be demonstrating this on a 2017 MacBook Pro with High Sierra. If you know such a USB drive is in your future, but do not want to toy with the terminal, you’re in luck–the process can be done with the help of a handy GUI. We’ve already covered how to do this from the command line (See How to create a bootable USB to install OS X), but I wanted to give those with a fear of the command line a way to do the same thing. With this handy flash drive, you can reinstall macOS, should something go awry. Stop! Before you do anything to that costly (and wonderful) piece of hardware, you should consider creating a USB installer drive for macOS. You bought that shiny new MacBook Pro or iMac, and you’re ready to start working. With the help of a GUI, Jack Wallen shows you how to create a handy flash drive for macOS. Your new Mac doesn't include an installation media. How to create a bootable USB installer drive for macOS

Universal usb installer for mac