Hello! 
I'd like to start making some custom Masters.  Is there any good way to make curved lines?

Hi Scott,
There isn't really a good method to create curved lines in SmartBuilder.  I assume this is for visual header/footer backgrounds?

Probably your best bet is to use Illustrator or even PowerPoint to create images (probably a PNG, so you can make use of transparency), and bring those in to SmartBuilder using an image object.  Depending on the nature of your design, you could even make a "negative" version, which has white sections surrounding a "knocked out" transparent section.  And then you can layer a shape behind that, and change the color of the shape as needed.

Or you can get creative with the shapes SmartBuilder provides.  If 2 shapes overlap, and they are the same color with no border, they look like a single shape. See the images attached for an example.