For this, you simply use a pencil and color in areas with dark/light shadows. This strategy is a slightly harder way of doing it, however the end result should be a lot better and look cleaner. The downfall of this strategy is that you can’t really do details with it, and its really easy to smear the rest of the paper. Lightly scribble on an area of your drawing, then you can rub your hand or a napkin on that area, and eventually the scribble will disappear and you’ll have an even and smooth gray patch on your drawing. Lighter shadows such as the top of the fuselage (if existent) can be used by 4B/5B, and darker shading like landing gear can be carried out by 7-9B pencils. For shading, you can either use a regular pencil if desired, 5B-9B pencils are also recommended.