Create a new document that is 600px by 600px. Set up your grid as shown below. Also, make sure that the Show Grid and Snap To Grid are both turned on.
Use the Ellipse Tool (L) to create a 200px by 200px ellipse. Use the Direct Selection Tool (A) and grab the bottom most point of the ellipse. Press the Down Arrow key three times. Pull the handles in to angle-off the bottom as shown below. Also, place a copy of the shape off to the side of the artboard, which we'll use later.
