You don't need a pricey tailor or a fancy sewing machine to make simple alterations to an ill-fitting suit. If a single-breasted jacket fits well across your back but gapes or bags in the front, converting it to double-breasted can elevate your look from sloppy to sleek. All you need are a few simple and inexpensive hand sewing supplies -- chalk, pins, buttons and thread.
Buy the buttons. If you can't find buttons that exactly match those already on your jacket, buy enough to replace the existing buttons as well. Make sure your new buttons are the same size so they fit through the buttonholes, and are compatible with dry-cleaning.
Determine the double-breasted overlap. Try on your suit jacket in front of a mirror while wearing clothes you'd wear under it, including the trousers. Pull the buttonhole side of the jacket across your front until the fit is just right -- sleek but not too snug. Pin in place.
Mark the first set of new button locations. With the jacket still pinned in place, use the tailor's chalk to lightly draw a small dot or X through each buttonhole on the buttonhole-free side of your jacket.
Cut off the existing buttons. Remove the jacket and carefully cut off the buttons and remove stray threads.
Mark the second set of button locations. Use tailor's chalk and a ruler to mark the spot for your new column of buttons at the desired width from the first set -- on the buttonhole side. Make sure they line up horizontally and vertically with your buttonholes. Note that only the first column of buttons will actually be buttoned; the second is just for looks.
Sew on the buttons. Double the thread and coat it with beeswax for strength. Run the needle through the buttonholes and to the back of the fabric several times, knotting on the underside to finish. Use the toothpick as a temporary spacer between the button and fabric so the button is not too tight.
Try on the jacket to check the look. If you're satisfied, brush away any remaining chalk residue.
Double-breasted suits look best on taller, thinner figures. They can have a widening effect on large-busted women and shorter or heavier men.