To install this signature in the Mail App using iCloud Drive on OS X Big Sur:
Open terminal and paste:
ls -laht ~/Library/Mobile\ Documents/com~apple~mail/Data/V4/Signatures/
Then paste:
open -a TextEdit ~/Library/Mobile\ Documents/com\~apple\~mail/Data/V4/Signatures/*.mailsignature
This opens a TextEdit file. Here you simply remove everything below the metadata:
Save & Close. Open the Mail App and you should see your new signature.