Cutting is actually easy. here's a couple of tips an upholsterer taught me.
If you don't have any old cushions, make patterns from cardboard or heavy brown packaging paper.
Cut the foam 1/2" bigger on each side than you want the cushions. Example= If your cushions to be 24" by 76" cut the foam 25" by 77" then sew the fabric to the dimensions of the patterns. It squishes the foam down a bit and makes for a tighter fitting cover.
To cut the foam, use an electric knife like you would use to carve a ham.
This makes cutting the foam real easy. Mark both sides, and go slow .
I have done cushions for my last two boats, and am getting ready to do the ones for my latest boat.
Patterned fabric hides a lot of stains. On boats, things get spilled when it gets rough. The patterns in a fabric can hide those stains. Use a fabric that has stain resistance in it when you buy it, or use a spray stain resister such as Scotch Guard. It's not as good as the fabric that comes with stain blocker in it from the factory, but it helps a lot.
On my last boat, I did all of my cushions for around $500 including foam.
I also made my own winch covers, sail bag, sail cover and cockpit cushions.
If you've never done any sewing projects for your boat, I wouldn't suggest starting with cushions, but go for the simpler things like pillows, winch covers, and other small things to get practice. There are plenty of books and videos out there to learn from.
I know that you're going to have someone else make them, but thought others could use the information.