Connect a 16GB or larger USB flash drive to your computer then launch the Mac App Store from your dock.
Navigate to the macOS Big Sur application page and click the Get button.
You may be brought to a Software Update window, click Upgrade Now or Download to begin downloading the update.
Once the installer launches, press the Command+Q keys on the keyboard to exit.
Open your Applications folder and select the 'Install macOS Big Sur' app. Press Command+i on the keyboard and verify its size.
If it's over 10GB, skip to the next step. If it's tiny (~20MB), you can either run it to install macOS Big Sur and then redownload the installer, or delete it, upgrade to the latest version of Catalina from the Mac App Store, then redownload the installer.
Once you've verified that you have the full installer, launch Terminal from the Applications:Utilities folder.
Note the name of your USB key.
Execute the following command in the Terminal window replacing 'Untitled' with the name of your USB key. Please note this will erase the entire USB key.
sudo '/Applications/Install macOS Big Sur.app/Contents/Resources/createinstallmedia' --volume '/Volumes/Untitled'
You will be asked to enter your Administrative account password.
Type Y and press enter to begin.
If prompted to allow Terminal to access your removable device, click OK.
After about 30 minutes the creation of your USB Install Key for macOS Big Sur will be complete.
You may now restart your Mac and hold down the Option key after hearing the boot sound. You'll be presented with a list of bootable drives. Select the USB install key to begin.
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