(This is for Photoshop only, not Elements, sorry.)
I was originally asked for this tutorial by somebody who was designing a fridge calendar. The link to the example calendar they wished to emulate has gone, unfortunately, but I’m including this tutorial here because it’s handy anyway.
First I filled the whole file with a dark red layer, then used the Shape Tool (U) to draw a rectangle at the top of the design. It’s important to make the rectangle quite a bit bigger than it needs to be:
I chose the Pen Tool, then clicked-and-held on it to get the flyout options. I chose “Add Anchor Point”:
I added a point at each side where I wanted the high and low point of the curve to be. This is done with a single click on the border of the rectangle. (You could turn on grids to help you if needed):
Then I moved one point up a bit, and one down a bit:
I went to Layer>Layer Style>Stroke, to add a white line:
And that’s all! Now it’s ready to act as a clipping mask for a photo at the top of the calendar, eg:
Finally, it’s ready for the calendar in the red space below.