Curves, Curves, Curves

This tutorial will cover some of the basics in making curves for banners.  
Create an image. I made a new 69x69 image and used the Rectangle tool to make it blue.
Select the Eraser Tool on the Pallete. Then navigate along the top bar to create a new brush.
Set the diameter of the brush to 138 and the hardness to 100%.
Now go back to your image and place the top of the eraser on the top-right corner of the image and the left side of the eraser on the bottom-left corner of the image.
Click, and you'll erase your image to get a beautiful curve.
This isn't the only way to make a curve, but it's one of the ones that's worked best for me. Experiment and you might find a better way.  
Once you've completed the basic curve, you can add images. You can place images on top of your curve, and erase whatever sticks out. Or you could place an image on the curve to make it look more than just a simple curve.  
You could even get a lot more daring and create a completely different curve.