Protractor / pencil on a string for perfect segments of a circle. Draw it out on cardboard or butcher paper, check to see if you'll scrape your nose on the way up. And check out skatepark ramps, other people's designs, jersey barriers, etc. for references.
For cutting, use a jigsaw, right?
Basically just do whatever's clever, but I'd draw it out on cardboard first and maybe even make a mock-up. Just go to a recycling dumpster and cut your curves, make a floor and deck, and then make a fort and invite all your friends over. Making out with babes in forts is awesome. Call me when you're done cuz I wanna hang out.