Curves are very natural and organic looking, and our eye is very drawn to them. So while I can appreciate linear lines in some designs, I always come back to the curves.
Often these curves are on flaps, which end up being a focal point of the design. Making the curves of the flap as smooth and as identical as possible is going to help a lot with the finished look of your piece.
So here are some tips geared toward sewing curves that look great!
True your curves by trimming each side of the piece to an identical shape before you begin sewing:
If you have a particularly difficult curve and just can't seem to get it right, trace the stitching line around the curve using a scallop rule or other curved item:
After clipping or notching curves, trim away about half the allowance so there is less fabric vying for space inside the curve once the piece is right side out:
If you have a lot of layers, you probably need to grade the seam. Grading is done by trimming the layers successively shorter, but leaving enough of fabric for the stitching to have something to hold on to (at least 1/8").
The photo below shows a straight seam, but demonstrates the concept of grading. You can see the batting is cut to within 1/8" of the stitching, then the lining is trimmed about 1/8" smaller than the exterior fabric (that one is left intact):
In clothing construction when grading you would trim the seams of any facing pieces first, but in accessories grade where you can. For the flat items made with my patterns you would grade the pouch and lining back/flap piece, and leave the exterior back/flap piece intact.
The grade looks tidy in the photo, but in real life the places you trimmed might look messy and choppy. That is ok, just get rid of some bulk.
If you happen to trim too close to the seam, go back and stitch in a new seam to reinforce that area.
Once your piece is turned right side out and ready for the final pressing, examine the seam around the curve.
Sometimes even though you pushed the seams out neatly from inside, once you remove whatever implement used to push the seam out, it will collapse back inward. For these sections you can massage the seam out by rolling it between your fingers until the fabric falls into place.
If the finger massage method doesn't push out all the caved in areas, then use a needle to carefully pull the fabric up.
Once you've got the curve looking neat, press the piece and then let it cool so the fabric sets in place.