OK,
Create a document to the appropriate size. Set up some center guides.
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Fill with appropriate color of choice for your BG. In this case....R 234 G 234 B 234.
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Add a vignette using an "inner glow" layer style set to a slightly darker color. Adjust to your liking.
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Create a new layer.
On the new layer, select the Rectangle Tool. Set the tool to "shape", then tun off the fill, and turn on the stroke, set the stroke to a desired color (black) and to 3px.
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Create a rectangle starting from the center guides and drag to an appropriate size.
note: Using the Rectangle Tool gives you editability..........you can make quick easy changes if needed.
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Now you have your basic BG for the example you provided above.