Now take the two pieces, fold them right sides together and begin sewing the seam. Use a scant 1/4" seam allowance, and match the curves up gently, stitching about an inch or two at a time, then match up a little more, and stitch a little more.
Be careful and try not to stretch the fabric, just ease them together, bit by bit.
Press one direction or another. Do not press open.
This technique is great for water or sky backgrounds! I used it to create the sky background for my Kula Sunrise quilt.