Since I run into this all the time and most of the articles I find are filled with ads and unnecessary information, I’ll post this here
First, make sure you have the Big Sur beta downloaded via Software Update. It should show up in your Applications folder if it all downloaded correctly:
To create the bootable USB drive, for the macOS Big Sur Beta (note the file name, it may be different for subsequent Big Sur releases), this is the command to run via the Terminal:
sudo /Applications/Install\ macOS\ Big\ Sur\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/myDrive
Replace myDrive with the actual name of your USB drive. If the command executed properly, you will be asked to format the contents of the drive. Say “Y” and then the formatting + copying will start:
Enjoy