Sometimes seamstresses get confused about exactly what a French Dart is. Research states it quite simply. A regular dart is straight. A French dart is slightly curved. Also, a French dart usually begins near the waist line or a little above or below, but typically, it is a long, curved dart as shown in the following illustration.
A student a had once chose to change the bust-fitting dart into a French Dart for her Tank Top project.
You'll find complete directions for designing this French Dart on page 25 of the Dress Kit Instruction Book.
Notice that the bulk of the fabric for the folded part of the dart has been removed and that you end up with a cutting line that curves up toward the dart tip.
When stitching this dart, you may want to add a stay tape, like a narrow clear elastic/plastic tape, since you are stitching the bias of the fabric and this will prevent the dart from stretching out of shape as you sew. Make sure to press this dart over a tailor's ham. And if the fabric frays a lot, you can always serge/overlock the raw edge of the seam allowance.
Everything about this tank top fit so nicely when she was finished. Well done!