May: The intention is each month you should be doing what I wrote about in the article at the beginning of the month. So start as soon as you like, and post with progress or questions. By May, you will (hopefully) be ready with a mockup and I'll talk about fitting and work though that with folks here a lot.
Spinsandknits: Metric! Super duper extra metric. Centimeters, to be precise. That's why the metric ruler is important (in the materials list). Trust me, that would become very obvious if you started working it up in inches (ask me how I know).