Forming a spherical form in one point perspective
Below is an example of a sphere derived from a cube.
Below is a step by step approach to building a three dimensional sphere.
Below is a close up diagram of how to find the circles within the squares.
Once you build all three circles you may begin the cross contour process.
This may take you some time, effort, and frustration at first, but when you are complete the exersize you will have a solid understanding of form in space.