I'll show you 3 very simple and common borders.
I'll start with the most easy.


Border number one, The simple Black stroke
Make a new layer over your finished signature and go to select>select all. As seen here.



Go to edit>stroke.
Now follow these directions.



Your final result should look like this.



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Now the more complex yet better looking in some circumstances, 3 pixel stroke

As befor go to select>select all. As seen here...



Go to edit>stroke.
But this time follow these settings.



Ok,follow the 1st step again.Select>select all and follow these instructions



Replacing the orange color the with main color of the signature.
And your final result is this



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Now for the third type of border which is better in grunge style.

Once again hit select>select all. As seen here.



Go to edit>stroke and follow these instructions



Set the Blending mode to Overlay [not NORMAL].

Hit select>select all again and go to edit>stroke (this is the last time i promise) and follow these instructions



now your final image should look like this