This tutorial is an important starting point for learning blending and contrast, and to get used to oil paints before you try more complicated projects. Keep it simple by only using back and white. Use the photo below for your reference image.
Steps 1 and 2
1. Mix a little black and white paint and some oil medium. Draw the outline of the sphere and its cast shadow with paint mixed with enough medium to make it runny and easy to move. Use a pointy brush, like a small round.
2. Paint a dark crescent with black for the deepest part of the shadow. Paint the cast shadow with dark gray and a darker proximity shadow (where the object would touch the surface it sits on.) Paint the light side of the sphere white. (Mine is a little darker so you can see it.)
Steps 3 and 4
3. Surround the dark crescent with gray. Paint a drop shadow in the background so that the foreground is lighter and it grades to a dark gray at the top of the canvas. Keep your individual color shapes on the sphere separate. When you blend (Step 4) you want to keep them like they are. You are only blending the transition space, or where the shapes meet.
4. You want to blend everything while it's wet. Use strokes going in all directions. Don't over-blend or your contrast will disappear. You want your darks dark and your lights light and a smooth transition in between. It's important to smooth the meeting place where the object and background and shadow touch each other. The cast shadow can have a blurrier edge than the object, especially as it gets farther way from the sphere.