You may have heard some other ways of doing these boxes, but this is just how I make mine.
1. Open up the image you wish to add a screened box to.
2. With the rectangular marquee tool, select the area where you would like to place your screened box.
3. Make a new layer (Layer > New > Layer).
4. On the new layer, fill that box with a light grey. I used #B7B7B7.
5. Set the layer blending mode to "Screen" and set the opacity down a bit. Depending on how dark or light your image is will affect this. I usually stay within 40-60%.
To add a border, you can simple zoom in and use the Line tool on a new layer.
My final result:

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