Generally formal dresses are boned, so that while you're dancing the night away, you can throw your hands up and SHOUT with confidence.
I'll be using boning with casing, strips of firm plastic encased in fabric. You can purchase this at Jo Ann's in white or black. It comes on a roll, so you'll need to give it a press with a warm iron to straighten it out when you get it home.
We'll be applying out boning to the wrong side of the lining fabric. I've already sewn up my bodice lining. You generally want to apply boning in a few places around your bodice, on the seams where your bodice pieces are already sewn together and the seams pressed down.
To get the boning to length, I just laid it out on my fabric and marked the end of the seam on the fabric casing. Very scientific. Cut the whole thing (plastic and casing together) to length. I've used wire clippers in the past, trying to save my scissors, but honestly scissors worked much better. Just be sure not to use your fabric scissors! (Yes, my scissors are labeled. You're jealous.)
Now we need to cut the actual plastic boning to be shorter than it's casing. Slide the casing back and cut about 3/4" off each end of the plastic (1.5 inches total), rounding the edge so there are no corners to poke you. This is so the part that's on the seam allowance doesn't have plastic in it, so you can sew over it when putting your lining and fashion fabric together. If you're using a different seam allowance than 5/8', adjust this cut accordingly, to be sure that your seam allowance will be bone-free. When in doubt, cut off a little more rather than less. You don't want plastic in your seam, it will hurt your sewing machine and make your bodice lie all wonk-a-doodle.
Time to sew! Line up the end of your boning with the top edge of your bodice. You want this to be centered over the seam (in this picture, it looks like I'm off center, but that's actually my french seam pressed to the side). Put a pin through the seam allowance just to get you started, and then hold the boning straight as you go. This will be a long easier than trying to pin it all the way down.
Stitch down one side of the boning, using the factory stitching as your guide. You want to stitch close to but not on the plastic middle.
Stitch all the way down to the opposite seam allowance, plant your needle and turn the fabric, sew across the end, then turn again and sew back up the other side.
Sew all the way around the boning, on all four sides.
Now use your fingers to wiggle the plastic into the center, so that there is an even 3/4" of empty fabric on each end. Throw down a couple of stitches to secure it.
Et, voila! When you sew this to your fashion fabric, remember that the boning is the wrong side of the lining. The bones will end up sandwiched between the lining and the fabric. staying out of sight and keeping your dress smooth and slouch-free!