3d sketch is similar to an illusion and now-a-days hold a high interest among many a people. Although it’s not that simple but it’s not even too tough. A bit of practice, creativity, observation and obviously time, will make you perfect.

Few important aspects to consider while making a 3d sketch are shadow and it’s different shades, reflections,midtones, strong contrast, cast and highlight where it will be important. For shading, the pencils 2H, HB, 2B, 4B and 6B are recommended. Practice shading with greyscale value chart will be effective.
GreyScale Value Chart
GeryScale Value Chart 2

Now the even more important aspect is the perspective – scale and size in space.
Using guidelines and outlines are effective. The most common method of practicing perspective is drawing the corner view of a town cross street. Marking vanishing points and drawing lines are suggested.Corner View of a Town Cross Street - Vanishing Points
Corner View of a Town Cross Street - drawing lines