You make a file, Win Notepad is ok as we're not posting HTML or RTF
here, just the text, not the two hyphens above it - Thunderbird makes
those. Save the file where you can find it and then tell Thunderbird
where it is: Tools - Account Settings - choose the account you want to
use the sig for - check the box "Attach this signature:" - and either
type in the file path or click on the Choose-button and find your file
that way. Hit OK at the bottom of the window and - Voilà - you have a
sig file.

If my explanation isn't clear, feel free to drop me a line by mail or
over Y-Msgr.