My second step was to define the shape of my dish. The general shape is called a parabola (or technically a parabaloid because it is a 3D surface created by rotating a parabola about its axis of symmetry). Parabolas are shaped such that they reflect rays (in our case, sunlight) to a central focal point. They can be described (and graphed) using a mathematical equation:
y = x2/(4f) Where f is the height of the focal point. For example, I could make a parabola with a radius of 2 feet and a focal point height of 1 foot by using the equation y= x2/4 and calculating from x=-2 to 2.
Another way of thinking about your parabola is to use the equation FD = R2/4 where D is the depth of the dish from the center to the rim and R is the radius of the rim.
Using my equation (I used y= x2/4 and calculating from x=-2 to 2), I calculated about 20 (x,y) points between 0 and 2. (I didn't worry about units, because I was planning on changing the scale later.)