Dave,
Buy some good quality Cotton Sheeting, soak it for a few hours in COLD water, then hang it up to dry, don't try to squeeze or wring the water out, just let it dry on its own.
When its fully dried, soak it again, do this two or three times to fully shrink the fabric.
Cut the sails out using the edge of the sheet as the long side of the Sail - - the Leech, this will give you a nice formed edge to the finished sail.
While the fabric is wet, do not stretch or pull on the fabric as this will cause your finished sails to go out if shape.
Sewing the sails MUST be done using only Cotton thread, manmade threads will not "move " with the sail and will create wrinkles.
The sails on my schooner were made this way and are now 17 years old.
John, :) :) :)