The side panels are flat-topped triangles that are later become modified during the fitting. The smallest flat top edge of the gores will form the armhole; and after the fit is completed, the upper portion of the gore resembles a classic Coke bottle.
The following measurements are needed for cutting the gores:
3) underarm to the floor: (Gore length __________)
For these pieces, lay the remaining fabric in a single layer, then fold each side to meet the selvage edges in the center, leaving a fold on each side. Measure the length of the gores on the fabric. Again, remember to add the same amount extra for hem length, seam allowance, and fudge factor (4"). Mark with chalk. Cut away the remaining fabric from the end of the gore length.
Now, measure the width of the tops of the gores (5) from either side of the fold. Mark with chalk. Then go to the other end and measure the same number from the two selvage edges with a chalk. With a straight edge, draw a line from the mark near the fold to the mark near the selvage edge. Repeat on the other side of the fabric. This should create a drawing of four triangles with flat tops.
For the mock up, it is a good idea to slice the rectangles at the folds, cut the diagonal lines, and cut the center under the selvage edges. In period, there was a fashion for false seams, which are a folded edge simply sewn up with a bit of shaping where necessary, like a really long dart.
Front and Back Center Gores
There are now two regular rectangles (front and back panels) and eight triangles with flat tops (side gores cut). The only waste at this time is from the first cutting. This may be used for the front and back center gores, and sewn into the centers of the panels.
For the front and back gores, fold lengthwise the remaining fabric piece cut from the panels. Cut this cloth to the necessary length, i.e.: From the belly button to the floor plus 2-3 Measure 1" from one corner, then 1" from the diagonally opposite corner. Draw a line between the two marks. Cut on the line. When complete, there will be four smaller triangles, two of which will be joined along the fold. I usually put the wide gore in the back, and allow the split gore to go on either side of the front opening of the dress.