I have no idea where to start! I was thinking to trace one of my covers or fitteds for the size and shape. But had a few questions too!
1. Do I put elastic in the legs and waist?
2. What do I finish the edges with serge, knit trim or other?
3. What do I use to fasten? Wool cord to tie wings together or should I buy plastic snap?

Thanks for the help!

You really can do any of the above. If I'm making a wool wrap, I usually make it two layers. You can serge or turn and top-stitch. Either way. I use elastic in the legs and waist on both types of covers.

It can be pricey to get nice snap equipment and I'm not sure how well the Joann's stuff would hold up over time. You could probably do the tie idea or use aplix or touch tape (good quality velcro). I've done both snaps and aplix, but tend to like aplix better for covers for the fit.

I would suggest buying some fleece or something from Joann's and practicing on that before you cut into your wool! It helps to perfect your pattern first. Some people actually just use 2 layer fleece from Joann's for covers anyway, so if the fit is nice, they'll probably actually be usable!