Step1:

Start out by opening photoshop and make a image of any size. when choosing the background color try avoiding choosing the color white as this will help the cubes appear more clear!

Step2:

In the menu go to filter > render > 3d transform. this should bring up a window that looks alittle like the one below.

Step3:

Ok now you have that window up... in the toolbar to the left you will see a button with a image of a 3d square.. click that. (the one in red above). in the space in the middle start drawing random squares. (below)

Step4:

After you have spent several hours drawing your random squares select the tool that has a circle with a arrow rotating around it (in red above). hold you mouse/cursor down on the image and move it around... the image should move around and at some point you will find something you like! (below)

Step5:

Click ok when you find something you like! when you are rotating the image and you only see the outline of just the blocks just keep rotating it you will eventually see the shades of gray making the 3d cubes... enjoy.

Addons:

You can also combine shapes by selecting the sphere or cylinder tool to make something else!