How do I put a personal signature at the end of an out going email?

and other email programs all have different ways.





Be more specific !





(I use Mozilla Thunderbird these days, and I must say it's a PAIN for setting a sig - using an external text file system - incredibly clumsy).
How do I put a personal signature at the end of an out going email?
On yahoo email, there is a check mark box at the end of your message box