Well...
Basically U need a software that can edit an image, that I recommend U use one of those guys say (PhotoShop or GIMP >> I usually use PhotoShop instead)
1. Choose your image to become your primary image of the signature
2. Make a canvas that fit the size of signature U want. If U want, U can also resize or crop it from your primary image
3. Adding some effect if U want or some text
So, that is for the basic.
And U need some knowledge of the software of course.
If U really want to make one, U should go to the tutorial section here :
http://forum.nihonomaru.com/tutorials/
there are plenty of nice people who make tread "how to make signature"
Well then, Good Luck!