This is just one way of making a one-off kite with curved panel seams. This approach is a bit imprecise but fairly quick for someone with a steady hand, and eliminates cutting separate patterns for each panel along with the sail template.
Drawing the seam lines
The seam lines are drawn on the cardboard half-sail template with the aid of a large hand-cut French curve or a spar with an adjustable bow-string. Larger radii look better from a distance than tight, curly ones, and are much easier to sew. Keep in mind the effects of the finished sail shape due to standoffs or billowing from wind pressure. I like to try to keep the seams as short as possible where they cross the allowance for the trailing edge hem. This is unimportant if a separate hem strip is added. Once the lines are laid out clearly and precisely, they are cut with sharp razor knife in a smooth sweeping motion. The cut is then refastened with bits of blue masking tape if another seam intersects with it. If not, the piece can be set aside.
The fabric weave should be aligned with the nearest spar or edge. Put another way, the fabric along the leading edge should have the weave parallel to the LE, the center of the kite should have the weave parallel to the spine, and the fabric along the trailing edge should be parallel to the TE. This has to be compromised in actual practice to make a kite, but the goal is to keep the tension on the sail from distorting the fabric, since tension at an angle will stretch it out of shape. Most kite plans indicate this alignment with a patch of cross-hatching on each panel.
Mark the fabric
Lay the fabric out as flat and smooth as possible. I use weights around the edges and let the fabric settle down for a while before marking and cutting.
A flat washer makes a good guide for the seam allowance around the panel template. Select a size that gives the proper width and roll it around the template with a soft pencil in its hole. A scrap of cardboard with a pierced hole works as well. Cutting the fabric with the hot-cutter in the washer is, for me, too difficult.
The overlap of adjacent panels can be one or both panels. leaving the full seam with on only one edge seems easier. I use an 8mm to suit my machine's three-step zigzag.
Cutting the panels
Two layers of fabric can be cut by stacking them under the template, but may stick together at the edges and ruin them. If you make a mistake here, two panels will be ruined instead of one.
Using the panel templates as cutting guides involves sliding them to match the pencil line, which may be 8mm away from the template edge. By trial and error, the template can be re-positioned and moved a few times for each cut and not interrupt the smoothness of the cut edge enough to be noticeable.
Each seam is cut in a few steps with a cutter of the right temperature to just cut the fabric. The center panel fabric is cut in one piece to eliminate a seam along the spine.
The cardboard template panels are now reassembled with tape to guide the assembly of the seams.
The fabric panels are joined in a way to make the sail aerodynamic and look good. Dark colors usually look better overlapping light colors. I like to "shingle" the seam toward the bottom of the sail's face in an attempt to reduce drag. Usually only one or two seams are tacked and sewn at a time to ease feeding the sail through the machine.
Tacking, taping or basting
The seams can be held for sewing by pinning, gluing, or with two-sided permanent tapes. Some of these tapes eliminate the need for sewing, sometimes by heating with an iron. They are used to assemble spinnakers and are quite strong, but outweigh the stitches they replace. The idea of putting an iron near my kite has kept me from trying this. Pinning or basting with thread makes ugly holes in kite fabrics, and basting stitches have to be picked from the final stitching.
I use glue, although sometimes the glue line shows through the fabric, and overuse will gum up the machine's needle. Glue-sticks are handy and cheap, but Fabri-Tac clear glue is stronger for a more solid bond, and has become my favorite.
This photo shows my way of holding fabric in place with hand planes
while gluing the seam with Fabri-Tac.
Sewing the seams
Usually sewing the panels to the center panel and working alternately toward the tips will allow folding the sail in half to check symmetry as the seams are done. Remember that the seam overlaps will be mirrored between the two wings.