Step 1: Select the rectangle function
Step 2: Create a Square by holding down the shift key and dragging your mouse
Step 3: Navigate to Effect :: 3D :: Extrude & Bevel
Step 4: Click on ‘Preview’ so you can see what you’re working with
Step 5: Increase the Extrude Depth to roughly the same size as the square you made (600pt by 600pt in this example)
Step 6: Click OK
Step 7: If you would like the ability to edit each side of the cube, go to Object :: Expand Appearance.
Step 8: Click on the object and ‘Ungroup”