Sandwich approximately 3/4" the fin between the two pieces of blue foam and sew the pieces of foam together. Use the Zig Zag stitch on your sewing machine since it makes for a better, more stable seam on the foam. Start sewing 1/2" from the edges of the foam since you will have to fit the pieces of foam together.
Since Jersey's measurement was 12", I didn't have to trim the blue foam down. If your dog is smaller that the total length of the craft foam, this is where you make adjustments.