Download the application Disk Maker X to make a bootable copy of MacOS Catalina for other machines.Once the download is finished, the installer will open.Depending on your internet connection, this may take a while. This will open Software Update in System Preferences and begin the download of the MacOS Catalina installer. Once the App Store loads, type 'MacOS Catalina' into the search box in the top-left corner and press return and click the entry for Catalina to go to the download page.Go to the App Store, this can be found by going to the Apple Menu > App Store or click the App Store icon in your dock.

You will need at least a 16GB USB flash drive. To get the installer file for Catalina follow the instructions below. The average user isn't going to want to enter terminal commands, too much information. There is a much simpler way then what Neal posted.