A godet is simply a triangular piece of fabric that is generally inserted between two seams on a dress or skirt to create more fullness. You can make godets of the same fabric as the rest of the garment or use a contrasting fabric for visual interest. It's better to use a light- to medium-weight fabric for godets, as heavy fabric won't hang as well. It's fairly easy to alter an existing dress or skirt to add godets.
Use a seam ripper to open the side seams of the dress. Generally, it's better to have the godet start just below the hip line, or lower, so you're not adding extra bulk to the hip or torso. If the dress has princess seams --- seams that run from the shoulder to the hem of the garment --- you can open those as well, depending on how much fullness you want to add.
Measure the length of the opening from the top of the godet to the hem. Add 1 inch for a hem allowance. Make a note of this measurement.
Lay out pattern paper on a flat surface. Pattern paper is covered with a grid that has 1-inch squares. Measure the seam allowance of the dress and double it. The top of the godet needs to be the same width as the doubled seam allowance.
Place a ruler on the pattern paper and draw a horizontal line the same length as the doubled seam allowance measurement. Mark the centre of that line. Draw another line from the centre of that line down the length of the godet, which is the same as the measurement you took in step 2. Set the ruler on the top of the centre line at a 45-degree angle, and draw a line down to the measured length. Connect the ends of the tops of the lines with the ends of the bottoms of the lines. The end result should be a triangular piece with a flattened top. Curve the bottom of the pattern piece as necessary to match the curve of the bottom of the skirt. Cut out the pattern piece, using a pair of scissors.
Fold the fashion fabric in half lengthwise with the right sides together. Set the pattern piece on top and pin it in place so the centre line is parallel to the grain line of the fabric. Cut out the fabric. Repeat this step as many times as necessary to create the number of godets you will be inserting.
Fold the hem allowance under and then under again, and press it in place. You can either topstitch it with a sewing machine or hemstitch it by hand, depending on how the hem of the original garment is finished.
Pin the godet to the dress, matching right sides to right sides of the seam allowances. Do not simply pin the whole godet to the dress and sew; the godet will be wrong side out. Do one side at a time and make sure that when you are finished all of the seam lines will be inside the garment and the godet will be right side out. Make sure the seam allowances and bottom edges match correctly. Use a sewing machine to stitch the godet to the dress. Always sew from the top to the bottom to avoid bunching. Press the seam allowance. Repeat this step for each godet and each seam.
You can use this method to add godets to a skirt or even to a pattern you are altering.
If the weights of the fabrics used are too different --- for example, inserting chiffon godets in a heavy wool dress --- the fabric may not fall evenly.