One thing I did to keep it simple: Eliminate the piping. I like the clean look without the piping. It also looks like it would make the sewing more difficult.
The Sailrite instructions have you make the pattern exactly as big as the foam. Then, when you put a 1/2 inch hem in, the cover will be slightly smaller than the foam, resulting in a wrinkle-free fit. I found that it was just a little too tight. Next time I think I would cut the pattern just a hair bigger than the foam.
I made a couple of throw pillows (of same fabric and thread) b/f I started on the cabin cushions. That functioned as a shake-down for my new (cheap) sewing machine. I had not done much sewing since junior high home econ class.
We went with a solid color b/c we think it looks better than pattern, but, it also makes mistakes less obvious.
There are some on-line fabric stores that sell roll-ends for less than normal price. They also sell small samples for $1 each. We found that some fabrics were stiffer than we would have guessed. It is also difficult to tell what the color and texture really is on the computer screen.
I bought my foam off of e-bay. The auctions usually specify exactly what density the foam is etc. I did a mock-up of all the shapes on graph paper in order to minimize the amount of waste for fabric and foam.