Try your dress on, pulling the zipper up as far as you can.
Measure the width of the top of the gap where the zipper won't close. Call this measurement "A."
Measure the length from top of the dress to where the zipper is stuck. Call this measurement "B."
Divide the measurement "A" in half. This new number will be the width of each gusset.
Create a pattern for the gusset. Draw a vertical line down the center of a piece of paper. An inch from the top of the paper, horizontally mark out the width of your gusset so it is cut in half by the vertical line. Draw a line from each end of the gusset width to the center line, that is the length of measurement "B." You should have a triangle.
Add 1/2 inch on each side of your triangle for seam allowance.
Pin the pattern to your fabric.
Follow the pattern to cut out a gusset.
Repeat steps 7 and 8. You now have a gusset for each side.
Unpin the pattern from the gusset.
Mark the 1/2-inch seam allowance on each gusset.
Turn your dress inside out.
Undo the side seam with the seam ripper. Only undo as many stitches as the length of measurement "B."
Pin the gusset over the opening, the point of the triangle facing down. The seam line of the dress should match up with where you marked the seam allowance on the gusset.
Stich down the right side of the gusset.
Stitch down the left side of the gusset.
Fold over the seam allowance at the top of the gusset.
Stitch across the top.
Repeat steps 14 through 19 with the other side seam.
Sew the gusset in carefully in order to avoid bunching the fabric.
These instructions are meant for an unlined dress.