Zara blouse with three rows of shirring added to the waistline. The shirring was formed with elastic thread with the rows spaced 0.5" (13 mm) apart.
Tops and dresses that are full or boxy, lack a defined waist, or just need a slight change can be quickly altered with some shirring or smocking. To try this out yourself, you'll need elastic sewing thread.
Hand-wind the elastic thread on your bobbin and use regular thread on top. Be sure to sew with the right side of the garment facing up so the elastic thread will be on the wrong side. If you're sewing around the circumference of the garment, start and stop at one of the side seams (or both, especially if you think you may have to adjust the tightness of shirring). As you start and stop each row or each section of shirring, leave long thread ends and don't backstitch. After you have finished shirring, and adjusted the fit if necessary, tie all the loose thread ends.
If you haven't sewn with elastic thread before, practice sewing with it on some scrap fabric first to see how tightly you should wind it on the bobbin or if you need to adjust the tension on your machine. Keep in mind that the smocking will get tighter as additional rows are added.
Here are some ways to try this technique:
Add shirring at the bottom of the shirt (i.e., drop waist) for a blouson effect like I did with the Zara shirt and like these at Topshop and Urban Outfitters.
Add shirring at the natural waist or the empire waist of a dress or top like on this fine gauge sweater at Anthropologie, this Theory dress, and this C&C California tunic.
Add shirring to short flutter sleeves to turn them into puff sleeves like on this Urban Outfitters blouse.
Add a bit of shirring to the center front of a dress just under the bust like on this Vince sweater dress.