The Sway Back issue happens when there is too much excess fabric at the back around the waistline but the front fits ok. So you need to get rid of the excess out of your pattern so that it sits better at the back.
As I've stated in my other tutorials on Altering your Pattern you must sew up a Toile/Muslin of your pattern first so that you can fix all the fitting problems that are happening so that you don't get discouraged and end up throwing your garment in the corner never to be seen again. I know I've been there!
- Pin the excess out from the CB to the side seam. It will end up looking like a dart.
- If you find you need to take the excess out right round to the front then it's not a Sway Back problem it's just a Front and Back Length problem ie. The Front and Back length is too long for your body.
- Transfer that measurement to your pattern. It should end up looking like a dart.
- Fold the dart in and secure with tape.
- Draw a line down the back seam.
- Cut away the excess and transfer that to the side seam.
- Redraw the back seam line.
- Redraw the back dart.
- To do this measure from the top of the dart to the CB seam.
- That measurement should be the same for the centre of dart at the bottom.
- Mark in the centre of the dart and draw the new dart lines in either side.
That's it you are done. The Sway Back pattern alteration has now been fixed.