D'oh! I mean, D'John...
Look up video tutorials on
www.sailrite.com. There are tutorials to take you from creating a template from your cushion area (not from existing cushions, it's what makes this tutorial so great for you), to cutting foam, to making the fabric covers. They even have a fabric calculator, so you know how much to buy. The tutorial also discusses a breathable bottom mesh fabric.
If you or someone you know is reasonably good with a sewing machine, you should do fine. Someday I may pony up for a Sailrite sewing machine, and then I envision cockpit cushions in my future...