Step 1

Start with a square. Here I made a new document 300X300, created a new
layer and filled it with a gradient

Step 2

Press Crtl/Cmd+T to enter free-transform mode.

Right click/Cmd click… when the dropdown menu appears, select SKEW

Click on the right middle square and drag up until you get your desired
angle. Press Enter/return to apply the transformation.

Step 3

Duplicate the layer

Step 4

Enter free transform again (Ctrl/Cmd+t) Right click and this time select
FLIP HORIZONTAL.

Drag out the copy to the left. Hold down the shift key to keep the baselines
aligned.

You may also want to make this side a bit thinner by clicking the left
middle square (node) and drag it in a little bit.

Step 5

Create a new layer and make another square and fill it with the same
gradient, this time make the lightest color at the bottom left.

Step 6

This time while in the free transform mode… when you right click/Cmd
click select DISTORT

drag each corner until you get a good match. Be sure the corners fit
nice and snug. This step will take a bit of practice to master.

Step 7

Finally select the top of the cube.

Image>adjust>LEVELS slide the middle triangle over to the right
to darken up the mid-tones. Repeat for the left side. This step adds more
realistic lighting to the cube.

Step 8

Ta-da A complete cube. Don’t limit yourself to just cubes. You can use
this effect for all kinds of shapes. You can even construct a cube out
of pictures. Just use images instead of the gradients.