(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.