How do you make a signature with the cellphone Samsung Flight, When sending messages?

[?Mz.Randomnezz?]

Ok so my friend has the Lg xENON and she has her signature as $$Virgo Babe$$ And i have the samsung flight (Both r At&t), How do u make a signature with the samsung flight be cause I REALLY wANT ONE! This is for texting Messages by the way!
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