Vegantastic, the hood is actually really easy. Measure how much of the neckline you want it to cover, and that's your bottom measurement. Cut that in half from the back seam, and that'll be how big you need to make the bottom line of your hood pieces(in red in the badly done paint sketch below). The purple line is the opening of the hood, and make that much taller than you think you need. I did mine by putting my coat on, measuring from the point I wanted the hood to end from, and making it very slack to the center of the top of my head(I did right at my hairline, in line with my nose to make it center-ish), and that's that measurement. Then I drew a very slopey---technical term, that---line to do the back. The bigger the slope, the fuller the hood, basically. I tested it in muslin first, to make sure it would drape how I wanted, and paperclipped that to my coat, since pinning would leave holes in the vinyl.
Hope that helps!