Double needles are the best. They aren't cheap, but they are so worth it. You know how t-shirts have a great hem with two rows of stitching on the top and loopy stitching underneath? A double needle is a home version of that stitch. You can get a universal, jean, or stretch double needle. Universal is for woven fabric, jeans for denim fabric, and stretch for jersey/swimsuit fabric. If you're lucky and your store carries a lot, you can get different widths. This is what the stretch double needle looks like.
To use a double needle you need two spools of thread and a bobbin. I just threaded two bobbins instead of one so I could use one up top.
There's only one change when you thread it. See the arrow pointing down in the middle right? Next to it in the groove there is a metal piece that splits the groove in half. One thread goes one side and the other thread goes the other side.
Then each thread gets it's own needle to be threaded through. All the settings are the same as a normal straight stitch. (If you move the needle postition, don't move it so far that a needle will hit your foot!) The great thing about this stitch is it stretches but it's not a zigzag! And it always looks beautiful because the needle makes perfect stitches.
To sew measure where you want to fold over.
Fold it over and pin. I did mine at 1/2". I've seen it range from 1/2" to 2". Pin over the entire area you want to sew.
When sewing coordinate the amount you folded over the where you'll sew. I had a 1/2" allowance, so I moved my needle so that the left needle hit at 1/2". Does that make sense? You want to catch the edge of the allowance with the outside needle. (Don't mind the random stitching in front of the needle. That was previous stitching I did so I could wear the shirt before I got the double needle.)
This is what the hem looks like from the top. Looks good huh?
This is what it looks like underneath. I used a dark green bobbin so that you could see the bobbin thread. The bobbin goes back and forth between each needle which is why it can stretch.