Sewing French Seams is a great sewing technique to have in your back pocket for finishing raw edges when you do not have access to a home serger. It can also elevate your garment quality especially with different types of silks and linens.
Typically when you sew a bodice together, you place right sides together and stitch the seam allowance. In this case the seam allowance of the Sun Day Tank is 5/8'' (about 1.5cm).
When you sew a french seam, you will ultimately sew stitch lines per seams, so it will be a bit different.
You will first place wrong sides together, pinning as you need along shoulder seams and side seams.
Stitch shoulder seams and side seams at 3/8'' seam allowance (about 1cm).
Trim down all seam allowances to 1/8'' (.3cm).
Do this for all seam allowances!
Turn garment inside out.
Press side seams flat . . .
. . . and press shoulder seams flat.
Press seams so that they are situated at fold, and stitch 1/4'' seam allowance (enclosing first seam in second seam).
Side view of stitching french seam on shoulder.
Do the same process for side seams (dart will situate just fine in the french seam).
This is how the interior of your tank will appear when sewn with french seams: