STEP 1: Create a New Layer and using the RECTANGLE selection tool select a square(ish) area on the left side of the image. Now go to SELECT/TRANSFORM SELECTION you can now scale the selection area but we dont want to do that, we want to skew it, so go to EDIT/TRANSFORM/SKEW and drag the handle on the right in the middle downwards to create the front face of the cube.

STEP 2: Now fill the selection with a mid-grey colour, and create the right side by copying and pasting your left onto a new layer, go to EDIT/TRANSFORM/FLIP HORIZONTAL to flip it, now move it so it meets the right hand edge of the first cube face you did, (select it by CTRL CLICKING in its layer I called rightside) now fill this with a darker grey. I selected the top area by eye using the Polygon Lasso tool, I just picked 4 points then filled it with a light grey on a new layer. We now have each face on its own layer. Hope this makes sense :)

STEP 3: While I still had each face on a seperate layer I selected each one and lightly airbrushed (with a large brush) near the edges lighter or darker as you can see above,this just defines the edges better . Then I selected the top layer and merged it down using CTRL E, then merged that down again so the cube was on a layer of its own, as shown above. Next, I created a layer between the background and the cube and selected an area as shown and filled it with a gradient set to go from Foreground (black) to transparent, this creates a shadow as shown.

STEP 4: I then selected an area on the background layer using the rectangle selection tool and filled it with a mid grey colour. Now for the next step we need to select the cube so Press and hold CTRL and click in the area marked red above, this will select all NON-Transparent pixels on that layer, ie. THE CUBE :)

STEP 5: Now create a new layer and fill it with a colour, I chose orange (dont ask why) anyway, set the layer properties to "color" as shown in red, this basically tints the layers below with orange.

STEP 6:Flatten the pic by going to LAYER/FLATTEN IMAGE then you can save it as a BMP or JPG etc. I hope this was useful to ya, its simple but I wanted to show how I use layers, selections, fills, etc to create simple airbrushed looking pieces. See Ya.