Draw a full scale on sheets of ply on the ground(birds eye view), cut out treads, build in sections, straight, curved and platform, easier to move later. Use 1/2" ply use PL glue and screws 2x4 on edge as braces under steps and at section connections, join with bolts/washers/
wing nuts. dont forget the tread thickness in calcs. Here is a link to a calculator for sizing,
http://www.blocklayer.com/Stairs/StairsEng.aspx
use centerline of stairs as tread length ie: stairs 4' wide, measure at 2' this will give you a Total "run", figure out how high you need to go to get "Rise" and plug in you #'s to get the dimensions you need.
Use your full size template to get inner/outer tread lengths, skin with "bending ply" for smooth curves.
Not exactly an easy job but doable. If you have more questions, i can likely answer them, I was a framer/ Carpenter for 10+ years.
Edit:
Use ideal run on the calculator, it will in general stay close to 7/11. may change number of steps but you can add/subtract risers for good fit. As far as the stringer, in your case it would be just a big plywood triangle with a saw tooth cut out. Does the outside radius need to be curved also? If not it would be easier to square it off on that far side for framing, the the only curve you have to deal with is the inside radius. I'll post a pic.