From your Lotus Notes inbox (or just about anywhere)
click on TOOLS (on the same line as the button for NEW MEMO)
click on Preferences
click on Signature
and type into the box you are presented with.
when you are happy with the signature, click OK.
One word of warning, depending on the font you use, what you see in the box on the create a signature panel may not be exactly what you see on your email – spacing and so on.