How to create a bootable installer for macOS

1. Download macOS installer

• From Appstore 

2. Use the 'createinstallmedia' command in Terminal

• Connect the USB (recommend least 12GB storage USB)
• Open Terminal
• Type or paste one of the following commands in Terminal. Notice: USB is name of your volume (device USB)
• *If your Mac is using macOS Sierra or earlier, include the --applicationpath argument, similar to the way this argument is used in the command for El Capitan.

Catalina

sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/USB

Mojave

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/USB

High Sierra

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/USB

El Capitan

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/USB --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

Yosemite

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/USB --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

Mavericks

sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/USB --applicationpath /Applications/Install\ OS\ X\ Mavericks.app

3. Press Return after typing the command.

4. When prompted, type your administrator password and press Return again. Terminal doesn't show any characters as you type your password.

5. When prompted, type Y to confirm that you want to erase the volume, then press Return. Terminal shows the progress as the bootable installer is created.  

6. When Terminal says that it's done, the volume will have the same name as the installer you downloaded, such as Install macOS Catalina. You can now quit Terminal and eject the volume.

Source: collect & edit