Boning gives a dress or bodice that shape you're looking for. It provides support, curves and lines in just the right places without the need to constantly iron out wrinkles from daily movement. Boning comes in more that one type. Plastic is the most common but is also prone to taking a permanent shape when a garment is worn regularly. Spiral steel boning is much more flexible and forgiving than plastic boning. Choose whichever you prefer for this project.
Transfer all of the guiding marks from the dress pattern onto the lining of the partially constructed garment if you have not already done so. Make the marks for the boning easy to find without damaging or staining the fabric.
Remove your boning from the casing if it is readymade. This will make it easier to work with because you will sew fabric to fabric and won't have to contend with the boning as it curls.
Measure strips of the empty casing according to the pattern and pin them to the wrong side of the lining or your garment along the previously made marks. If your pattern calls for the boning to lie over a seam allowance, center it on the area so the edges lie along the seam and pin it securely.
Lay out the cotton and measure the length you need for each bone based on your pattern. Mark them and create channels by sewing through one layer of the fabric. For 1/4-inch boning you will want to sew casings of 3/8 inch. Cut these out with a 1/4-inch seam allowance on either side. Pin them to the garment.
Sew each piece of casing to the lining, following the stitching lines on either the store-bought casing or the casings you made. Sew both of the long sides and across one end. Leave the other end open to insert the boning. Try to ensure the ends of the casing do not get caught in the seam allowance if you are making a bodice or adding structure to a waistband. If the boning pokes into the seams, it will not lay flat.
Insert the boning into each casing on the garment, snipping the pieces just short of the open edge. Give them a round edge to prevent them from poking. Close the open edges of each bone by hand.
Set your iron for a heat setting appropriate for your fabric and set it for high steam. Begin ironing over the bones because they are probably curling slightly. The combination of heat and damp will cause the boning to take the natural shape of the garment for now, and when you wear it, the bones will conform to your shape unless you iron it heavily again.