I made all the fitted sheets for Insatiable the first. It is easy to do, all you need is a sewing machine, sheets to alter, and pins. I laid out one set of sheets on a pier in New Caledonia
Take a bench brush ashore to clean the place you're going to put the cushions
, or use clean grass on which to place the cushions
for the fitting with pins. After they're fitted, mark the darts at the corners (all four for the bottom sheet, bottom two for the top sheet, (or use elastic for the bottom of the top sheet).
I would suggest you use less expensive sheets for the first mockup if you have never sewn before. Then get some really nice ones, and incorporate what you learned from the first ones. That way, if someone leaves the forehatch un-dogged, you'll have some dry ones, ready to go.
More details available on request.