First off, you'll need a quick crash course on perspective if you want your cube to look real and accurate. Linear perspective is comprised of one point, two point and three point compositions.
Linear perspective was rediscovered by Italian Renaissance architect and artist Filippo Brunelleschi around 1420, after probable knowledge of it was lost in Ancient Greece/Rome.
It is used by many architects, draftsmen and landscape painters today. I will try and break it down as brief and simple as I can.
Here is one point. It's where you have 'one point' (vanishing point where all lines meet) and a horizon line (where the earth meets the sky). You start with the cubes face. It's basically a square.
ONE POINT PERSPECTIVE CUBES
Here is two point. 'Two points' on a horizon line. You start with a line that represents the corner of a cube that is closest to you.
TWO POINT PERSPECTIVE CUBES
Three point is only three points and no horizon line. This is because it's kind of like looking up or down a skyscraper. You're looking at either all sky or all ground. Not where sky and ground meet.
THREE POINT PERSPECTIVE CUBES (skyscrapers)
For this tutorial, I will be demonstrating how to draw a cube in two point.
Pick up a light graphite pencil. Preferably 2H or lighter, so if you mess up, you can erase it completely. *Feel free to use a ruler or straight-edge.*
GENTLY sketch out the contours of your cube. If you're not comfortable enough to freehand the perspective, use the horizon line and points like I showed you. Be sure to do it light enough to erase.
Now to start dropping the cube back into space. Put a light layer of shade over the entire cube, and the space around it. This will deminish the lines you just made and help make it look seamless.
To start giving the cube dimension, take a darker graphite (I used H), and outline ONLY the outside of the cube, and start shading in the background.
Here is what mine looked like after the first layer of background with my H pencil.
Next, add a second layer of shade with a slightly darker pencil. (I used a 2B).
Here is what it looked like after the second layer.
You don't want your cube to look like it's floating in mid-air. With your second darkest pencil (7B), give it a shadow, as if it's sitting on a table.
Going back to a 2B, begin building the inside. Notice how I leave one side alone, because that's where light is hitting it. *make sure it's the side opposite of where you put your shadow.
With a 4B, build the value on the darkest plane (the one with the shadow), while leaving the others alone. *Make sure to fill the planes solid and smooth, hiding any contour lines you've made.
If you want to, you can make it even smoother by buffing with blending stump or tortillon.
Thank you for sticking with me through this somewhat lecture and enjoy! Send me your cubes at firstname.lastname@example.org
*opps tip* If you ever make a minor mistake or unwanted dark spot, dab it with a kneaded eraser to lift the graphite without smudging the rest of it. Then blend over it.