First things first is to open your sig, i'll use this one:

user posted image

1. first off do the following twice: {LAYER>DUPLICATE LAYER}.

2. select the bottom layer and do the following: {LAYER>DELETE LAYER>LAYER}

3. select the top layer and do the following: {CTRL+I}

4. select the gradient tool, and change the settings to the following:



5. now starting form the top do tiny gradients all the way down.

6. If the sig is colored it might look a little strange, so...

7. Drag the top layer to the bottom.

8. Select the top layer and change the blend to {COLOR}

9. End result:

user posted image

Mpre examples:

Can you put this in the tutorial section please?