Make the sig you want, when it's completely done, make a new layer + apply image.
Now, make a new file be sure to make it transparrant, then drag the layer you made before on this new file.
Now you can start cutting, moving etc. And once you're done save it as PNG.
Sorta .. easiest way to do this.
Personally I don't really like them, simply because.. they usually turn out
H-U-G-E <=
Greetz, Miyo ^^