Start with 3 straight lines and then add 2 diagonal line from the middle line to the right and left lines
Next add another line above the lines below. Just do the same thing but make them look like they are going to connect eventually
Last to for the top surface just from the left vertical line just draw the line to the right side and the opposite for the other side
Last just make at least 8 boxes minimum, and start to manipulate the boxes into some thing like a lamps and dices to the more advanced like the PC
Start fineline all the boxes and no need to fineline the extra guide lines you have made. I'd recommend to let you use the 0.8 for the outer areas for cube and 0.3 for the inside. This is to let the outer areas stand out and lift from the page
The difference between the 0.3 and 0.5 or 0.3 and 0.8
First start with rendering the outer sides. which ever side is the longest, follow up the side with the stroke.
Next make the outer areas darker by adding layers on the sides and add color to the middle also and depending on the where the light comes if it come from the left side, make the left and top lighter and the right side darker. Do it for some shapes you want to do and try to spread the evenly.
Same thing make the darker side with more black pencils and the lighter side more lighter with less black pencil and more white China graph pencil. Last thing make the top surface lighter with the China graph pencil also.