help with technique

What is the quickest way to make this box pattern effect?

This is the way i'd do it.
- Make a new document with transparent background, and about 3x2 aspect ratio.
- then select the whole document, and deselect the edges.
- Invert the selection (Shift+Ctrl+I) and add about 1 pixel thick selections along the sides
(or more depending on the size)
- Fill the selection with white
- Then go "Edit - Define pattern"
Then its done. And you may fill it in to any desired picure

I'd say make yourself a pattern, like Ensomuly described.
For example, make a 60x45 pixel document. Use the rounded rectangle tool, and adjust the radius (I used 6px) until you get the shape you want. Draw the rectangle path. Go to the layers window, click the path tab, and Ctrl+Click on the work path (makes a selection from the path). Then press Ctrl+Shift+I, and fill that in with black. Go to Edit->Define Pattern and name it. Then go to your image. (Double click the background layer to unlock it, if applicable). Then create a new layer, and move it under the background layer. Fill it with white. Now, select the layer with your picture on it, and go to Layer->Add Layer Mask->Reveal All. In the layers window, go to the Channels tab, click on where the (eye) should be next to "(Layer Name) Mask" and then click actual layer to select it. Go to Edit->Fill. Under Contents->Use, select "Pattern", and select your custom pattern, and click ok. Still in the channels tab, hide the Layer mask. And that's it.