First, cut out the pieces you need depending on what style you're making. For the torso piece and bottom torso piece, cut 2 fabric pieces and 2 lining pieces.
To start, first put in a stretch needle! This will keep your fabric from getting holes.
Then sew together the casing on the fabric and the lining at 5/8". I sewed with a straight stitch because this piece doesn't need to stretch as it will be gathered.
Lay the fabric torso piece on top of the lining torso piece wrong sides together.
double needle for this next step but you can use a basic straight stitch if you like. I used the little screw in marker to find where 5/8" should be. Then sew down both sides of the seam. You are creating the casing between the fabric and the lining because you want the lining to stretch like the fabric.
stitching in the ditch. This separates the casing into two.
Now pin the outsides of the piece together and serge or zigzag all the edges together.
Starting in the middle use both hands to stretch the elastic to match the length of the fabric. I sewed with a wide long serpentine stitch, but you can use a zigzag stitch too. Don't use a straight stitch.
After you sew in the bra cups, you just need to sew elastic to the edges and finish the edge with a double needle. Then you have your growing swimsuit!
Let me know if this makes sense! I hope to see the suits you make!