I prefer to install macOS Betas on a clean partition rather than upgrading an existing macOS instance. Apple has documentation on how to do this with many existing macOS releases, but they do not specify how to do this with beta versions.
To create a bootable USB installer for macOS 11 Big Sur Beta 1, you’ll want to connect a USB drive to your Mac, erase the entire drive in Disk Utility so you have one partition named “Untitled”, and run this command in Terminal. Keep in mind, the account doing this will need administrative privileges:
sudo /Applications/Install\ macOS\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled
Terminal will ask for your current user’s password and create the install media.
Once this is complete, power down your Mac, and turn it on using the Option key on your keyboard. From there, select the installer, and you’re set for whatever you’d like to do, whether you want to install it on a separate partition, make it your daily driver, and so on.