Drawing a side in 2D :

- Open a new canvas.

- make the visible side of the box : here, I filled a square with a green gradation and past the text two times (a yellow layer and under another white one stretched with the filter ["Outline" / "Stretch"] )

For more details : text's tutorial.

- When it's done, select it with a rectangular selection and copy it.

Preparation :

- Open a new canvas.

- Display the grid ["View" / "Grid"]
A dialog box appears. Choose the size of the squares and check the case : "Magnet"... This will hook your selections to the intersections, and will allow you to work with an extrem accuracy.

Temporary drawing of the box :

- Draw the box with the layout's tools :
Make several selection "line" ou "broken ligne" . Highline them as one goes along (be carefull to the pensize...).

(About the tool "Broken line " : to end the selection, just right-click.)

- This drawing just serve to give you some marks for the rest of the work. It will be clear at the end...

Preparation of the collage :

- Paste the visible side that you've made during the first stage...

- The picture appears on your canvas. Place it approximately where you wanna put it.

- Right-clic on it and select "Free transform".

Transform and apply a side :

- Move each handle at the selection's corners toward the box' corners...

- Right-clic on the selection and "Paste as new layer".

- That's all...

Seconde side :

- Proceed exactly the same way for the second visible side...

...Paste / Transform / paste as new layer...

Finishing :

1) Change the light on the visible sides :
- Use luminosity and contrast ["Image" / "Luminosity and contrast"], to brighten a side (darken the other one the same way, if you want).

2) Make the inner sides :
- Add a new layer for each inner side.
- Select a side with the selection "Polygon" and fill it with a required color (try some grey...).
- Proceed the same way for the other inner side.

(About the "Polygon" : to end and close the selection, just right-click.)

3) The shadow :
- Draw the bottom of the box with the selection "Polygon" and fill it with black...
- Apply a Massa's filter ["Motion Blur (Natural)"].
Settings : pixels = 50; choose the angle according to the opposite direction of the shadow.
- Lower the transparency's layer : about 50% (double-click the layer's name)

You can make a self-made shadow. You can draw it and blur it the way you want... There's no recipe...

4) More :
- Give a texture to each layer (except the shadow !) with the Gonta's filter ["Effects" / "Chaos"]...
- Be reasonable with the effects (luminosity, contrast, texture...). It really looks better when you work with light shades !