I made both the hex tarp and the winter dream off the DIY Gear Supply page, both out of the seconds-grade polyurethane-coated camo.
I started with a string, a couple of thumb tacks, a marker, and a piece of silver-sided building foam/insulation, and made one parabola 6 feet long and 6 inches deep. I plotted out both patterns on XY grids, based on the plans and given dimensions. Then on my coated nylon, I marked the points taken off the XY grid. Then I fit my curve as best I could between the points--if two points were less than six feet apart, then the curve just wasn't the full depth of 6 inches. Since then I've done patterns where I needed a longer curve, but it's easy to eyeball it, just put the curve halfway between and then extend the lines on each end to the correct point.
I've always thought that a catenary ridge line looked pretty cool, but I did mine straight, and just cat-cut the edges. I was worried that if I cat-cut the ridgeline too then I might get floppy ends. Both the hex tarp and the winter dream, based on the plans, rig up pretty tight.