some of the tutorials my wife used are: [ link ]
[ link ]
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[ link ]
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The reason I am recomending the ones my wife used is because she is immensly better at digital artwork than I am. Just learn the techniques that they use in the tutorials and then apply them to signatures.
And btw, please try to stay away from the techniques that Deadrain, Ripper and DVDback are using at the moment. They are all producing the exact same style signatures over and over and over, & it is starting to all look like the same boring shite.

They are nice guys, but they need to start making sigs and not just cut/pasting pics & renders.