How to put a border

[40piggies]

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[Mark1]

There are a ton of ways to do it. But a simple way is to do it last. Flatten all of your layers. Then go to "layer" menu ( I think-- im not on my editing computer) near the bottom is "stroke layer" you can pick a color and size you want. Then resize if necessary.

[Microcosm]

I usually wait until the end, then (I have Photoshop CS3) I go to IMAGE > CANVAS SIZE.
I click the box that says RELATIVE, and set the canvas to extend on all sides by 10 pixels and the color to white. You can adjust it as you like, change colors, whatever.

[40piggies]

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