Technique: Rounded Pockets

Here is one way to get smooth, rounded corners
on patch pockets like these

First, you'll want to baste around the edge of the pocket. 
Use a long stitch length and do not backstitch at the
beginning/end. The stitching should be the same distance
from the edge as the amount you need to fold under. 
So, if you want to fold under 1/2",  baste 1/2" from the edge. 

Because you didn't backstitch, you'll be able to pull on the loose
thread ends--just one of them--to create a slight gather. 
This is going to help create a smooth edge. 
The rounded edge of the pocket will naturally fold inward some.

Here's how the front is starting to shape up.  You'll want to
adjust the amount of gather until you get a smooth curve.

Here's what the pocket (the other one) looks like once it's
been ironed and is ready to be attached. 
It's got a much nicer curve than if I had just tried to fold the
edge under and iron it, and I burnt myself less in the process :)
While sewing the pocket on the garment, you can use your seam ripper
to poke in any bits that look pointy, to make them smooth.
Once the pocket is sewn, remove any basting stitches that sho.

I appreciate feedback, so if anything is unclear or incomplete please let me know and I will do my best to fix it!

Technique: Matching Seams

It can be tricky to get seams to meet, like the seams 
between the ruffles of this dress when the front and back 
are sewn together.  This is how I like to do it.

Hold the fabrics right sides together, near the seam to be matched.
Fold the edges back enough to see both sides.

The key is to make sure the seams line up at the point where
the seam will actually be sewn, not just at the edges.  This
shows two seams meeting at my 5/8" seam allowance.  You may
wish to pin your fabrics together at this point.  I usually hold 
them firmly so they don't shift.

Set your machine to a basting stitch.

Baste across the seam.

Sew a little before and after the seam intersection.  You want to
make sure you sew enough to hold the fabric together well, but not 
too much in case you have to remove the basting stitches.

Check and make sure the seams match.  If they do, sew
the seam as you normally would.  If not, remove
the basting stitches and try again.