Create a new document of whatever size and aspect ratio you'd like. Unlike Photoshop, in Illustrator it's quite easy to change the document size anytime during our work. We can simply resize the vector graphics to fit the new dimensions. I initially chose a square document at 550 pixels by 550 pixels.
Since the process of creating our image is primarily just layering shapes with different gradients and fills over each other, most of the following steps will simply say what the size of the circle should be, what the gradient type is, the gradients direction, the colors used in the gradient, their color stop locations in the gradient, and the location of the center slider(s) in the gradient.
Make the first circle is 429px wide, with a -45 degree linear gradient, which has a 100% gray (at 100% stop) to 40% Gray (at 0% stop), and the center slider set at 50%.
Make the second circle is 423px wide with a -45 degree linear gradient, which has a 100% gray (at 100% stop) to 60% gray (at 0% stop), and the center slider at 50%.
