That right there is a very, very broad question. Your best bet is to take a look over at the
Constructing a Strap Shield page over on the
Geddon. That will answer 90% of your questions.
The only things that's really vague in that tutorial is the type of plywood to get. For you, I would recommend a ~28" round shield. For that, you'll want 1/2" CDX or 5-ply birch plywood. The CDX comes in 8' X 4' sheets, so your best bet for a single shield is to get some of the 2' X 2' 5-ply birch that Menards sells (I don't think Home Depot carries it). It's light, and will never break at that size.
If you have more specific questions, please ask, but that's that best "in general" response I can give ya.