Why do you want to be able to draw a sphere? Because from there it's not far from drawing an egg, or a human head. Here's the process in a few easy steps!
Charcoal, a black Conte crayon or a soft lead art pencil. It works with a regular HB pencil, but the darker the better.
Drawing paper or sketchbook
Kneaded eraser
Krylon or Blick matte fixative
Optional tortillon, stump, chamois or other blender
Step1
Freehand circle in charcoal
Draw a freehand circle. It's important to do this freehand because the more often you draw freehand circles by eye, the easier it gets. It doesn't need to be perfect or have a smooth exact outline, but don't go too heavy in fixing it.
Step2
Shadow and highlight spot are sketched in
Decide which direction the light's coming from. Draw a flattened oval for the shadow going off at an angle, and lightly draw a smaller circle on the face of the future sphere. I did this with a dotted line because it's charcoal, but you can just go lightly if you're using pencil. It doesn't have to be perfect or permanent, just mark an area opposite the shadow. If you look at a ball on the floor with a strong light to one side, you can see the spot of highlight falls opposite the direction of the shadow. If we had more than one object in this drawing, the light would have to be the same on everything, that would make it a realistic drawing.
Step3
Core shadow and deep shadow sketched in, the darkest shadows
Sketch in the core shadow on the sphere. Look for it when you draw from real eggs, balls or whatever. It's subtler than it looks in a drawing, but it is there. So is the reflected light right at the edge of the shadow. The core shadow does not go right to the edge of the sphere on the dark side. It curves around and either forms a crescent or a crescent with slightly flattened ends. I drew it in good and dark so it's easy to distinguish. I also darkened where the sharpest little dark line of shadow appears under the sphere within the shadow shape.
Step4
Halftone (sometimes called midtone) smudged in, cast shadow shaded
Blending the halftones. We have big white areas and a stark black core shadow. Let's smudge it. If it wears down too light it can be built up again. There is a little reflected light under the core shadow where light bounces up off the surface even within the cast shadow -- that's the oval one that makes it look like it's laying on a table. Halftone is the shaded area between the very dark core shadow on the sphere and the light highlight, which we will leave white. Very lightly shade in the cast shadow darkest toward the outline and lightest where it's right next to the sphere. This exaggerates the light reflecting off the sphere into the cast shadow. Doing these light and shadow effects bolder than life makes it art, it gives it a lively three dimensional look.
Step5
The finished sphere, a little shinier and darker with a smaller highlight
Finish by darkening the core shadow and blending it gradually into the midtone. Smudge the cast shadow texture smooth, and darken it by rubbing over it with your finger or blender after strengthening the core shadow shading. Clean up around the edges with your kneaded eraser. To soften the highlight, squish it into a rounded shape and press it on the center of the highlight, then lift. Spray your sketched sphere with matte fixative, or it will smear all over the page.
You can practice this sphere in a sketchbook, or start doodling it small with a pencil at work. Doodling an art exercise like this on scratch pads and phone pads is a good way to get so used to it that you'll never forget it -- and apply this kind of shading when it's a soccer ball, an egg or someone's head.
Spheres like this in strong bright colors are often used in pop art to contrast with other shapes and with the background.
Tips & Warnings
Shading with tortillons (one-ended cardboard blender) or stumps (two-ended blender) is a way to get softer grays in charcoal. Use the point to rub into a very dark area, then draw with the blender rather than the charcoal stick.
You can also shade a sphere with crosshatching, stippling or curving hatching lines that are darkest in the core shadow. Look at the AT&T logo for a sphere done with curved thick and thin hatching lines.
Wash your hands often while charcoal drawing, it's messy and finger smudges can give you unintended darks.