Here is how to create a USB install disk for macOS 10.9 and up.
1) Download the installation app from the Mac App Store. You will see the installation window pop-up after the download finishes — just close this. Now you will have the installer in your Applications folder. The application we need is highlighted below, it’s called Install macOS Sierra.

Install macOS Sierra
2) Insert an 8 GB or larger USB thumb drive. From Disk Utility (Applications -> Utilities), highlight the top level of the drive on the left sidebar (in this case it says SanDisk Extreme…), then click the Erase button on the top toolbar. Name the drive “USB1” if you are following along, be sure to use “GUID Partition Map” or the drive will not be bootable, then click the blue Erase button.

Formatting the USB drive

3) Open the Terminal (Applications -> Utilities), and run the appropriate command for the version you are creating:

For macOS 10.13 High Sierra:
sudo /Applications/Install\ macOS\ High\ --volume /Volumes/USB1 --applicationpath /Applications/Install\ macOS\ High\ --nointeraction
For macOS 10.12 Sierra:
sudo /Applications/Install\ macOS\ --volume /Volumes/USB1 --applicationpath /Applications/Install\ macOS\ --nointeraction
For macOS 10.11 El Capitan:
sudo /Applications/Install\ OS\ X\ El\ --volume /Volumes/USB1 --applicationpath /Applications/Install\ OS\ X\ El\ --nointeraction

Copying the installation files

4) Creating the USB install disk can take anywhere from 10-30 minutes depending on the speed of the disks. When it’s complete it will say “Done”, then you may eject the USB drive.