Excerpt from: http://www.craftsy.com/blog/2015/02/sewing-curved-seams/
You can make beautiful garments, bags and home accessories by sewing only straight seams, but if you want to tackle designs such as scooped necklines, circular pillows or rounded clutches, mastering professional sewing techniques, then you will need to learn how to sew curves.
Curves may seem intimidating because they require a bit more finesse when handling the fabric and the machine, but they don’t have to be!
The following tips will help guide you through the process of sewing smooth, even curves.
Tip #1: Draw the seam line on your fabric before sewing.
When guiding your machine needle around a curve of your fabric, it can be difficult to maintain an even seam allowance. For greater accuracy, it helps to measure the seam allowance manually and draw it onto the fabric with chalk or removable ink. Your eyes can then focus on the area of the fabric in front of the machine needle, instead of having to look over to the seam guide on the throat plate.
Tip #2: Focus on the area directly in front of your machine needle.
If you try to yank the fabric around too forcefully in anticipation of a sharp curve, you may turn your fabric too soon or pull your garment out from under the presser foot, which can lead to skipped or jagged stitches and uneven seams.
If you instead think of a curve as a collection of small straight stitches and focus on just a couple stitches at a time, then you will sew more accurately. Keep your eyes focused on the area right in front of your machine needle as your target.
Large or long curves will seem less intimidating this way. To guide the fabric around the curve, gently push the fabric sideways with your fingertips as the machine sews.
Tip #3: Shorten the stitch length
Sewing around tight curves is easier with a shorter stitch length because the machine moves the fabric under the presser foot more slowly. Smaller stitches also make a curve look more smooth, while long stitches can make the curve look more angular.
Below, the left curve was sewn with a longer 3.0mm stitch length and the right curve was sewn with a shorter 2.0mm stitch length.
Tip #4: Walk, sink and pivot.
When maneuvering around tight curves, sometimes the machine can’t turn sharply or quickly enough to stay on course. To assist the machine with this, use your handwheel to “walk” the needle through each stitch.
This gives you more control over where the needle lands and leads to greater accuracy. You can also use the handwheel to sink the needle down into the fabric, then lift the presser foot and pivot the fabric slightly before putting the foot back down to keep sewing.
Any time the fabric is bunching too much around the presser foot as you try to steer the curve, lift the foot so the fabric relaxes before you keep sewing.
Tip #5: Reduce seam allowances before sewing.
This tip is particularly useful when sewing a convex curve to a concave curve. A convex curve is a protruding curve (think of the ears on a stuffed rabbit) and a concave curve curves inward (think of the neckline of a dress).
When sewing a convex curve to a concave curve, such as when attaching lining to a facing or when piecing a quilt, you have to ease the convex curve’s edge into the smaller edge of the concave curve. As you can see, the raw edge of both of these pieces are different lengths, but the seam lines are the same length so they match up when sewn together:
Larger seam allowances therefore means more fabric needs to be eased into a curve, which can lead to puckers. If you are working with a pattern with 1/2″ or larger seam allowances, consider reducing the seam allowances to 1/4″, so it is easier to pin and sew.
You can do this on your actual pattern pieces for greater accuracy before you cut out your fabric, or you can trim down the seam allowances on the fabric pieces before you sew them together.
Tip #6: Match the centers.
If your pattern does not include notches along the curve to help you align the pieces, find the center of each curved piece then mark with chalk or a small notch. When pinning pieces together, match the centers as well as the two ends of the seam. That way, if you have to ease the fabric together, it will be distributed evenly.
Tip #7: Cut notches into a convex curve and slits into a concave curve before turning.
Convex curves have more fabric in the seam allowance than the actual seam line, so if you leave them as is and try to turn the project right side out, there will be a lot of bulk that bubbles up along the edge and looks unsightly.
You can reduce that amount by trimming close to the stitches and then snipping small wedges out of the fabric within the seam allowance. Be careful not to snip into your stitches! When the project is then turned right side out, there is less fabric being shoved against the seam’s edge.
Concave curves have less fabric in the seam allowance than at the actual seam line. Cut small slits into the seam allowance to allow that fabric to stretch and flatten out once the project is turned right side out.
Tip #8: Use a blunt pointed object to help smooth out curves.
Once you turn your project right side out, push out the curved seam with a dull pointed object such as a point turner, knitting needle or chopstick to smooth out the edge. Don’t use something too sharp or else you may poke through your seam line. Do this right before ironing.
Tip #9: Press, press, press.
The iron is a critical part of the process of sewing curved seams. When you first turn your project right side out, the curves may look bumpy or wrinkly. A bit of steam and weight from the iron will smooth out those lines for a more professional result.
Use a tailor’s ham to help press out curves in a garment, such as a hip curve, princess seam, collar stand, or curved dart. By ironing the seam over the curved edges of the ham, the garment will mold to that three-dimensional shape and fit better on your body when worn.