I've created this tutorial in Elements 9, and it's relevant to all versions of Elements and Photoshop as far back as I can remember.
I begin with a blank file:
... which appear at the left and top of your image window:
Then you can drag guides out from the rulers and place them in position. Here, I've positioned them 100px in from each edge:
Then I go to the Shape Tool in the toolbar (shortcut is the U key). By default it's on the plain rectangle shape, so I click and hold on that to choose the Rounded Rectangle shape instead:
Then I take a look at the Options Bar for that tool. The default radius for the roundness of the corners is 10px ...
... but that's too small for my taste this time. I change it to 50px:
It's time to draw my rounded rectangle. If a rounded rectangle was all I wanted to achieve, I'd simply draw it to the exact size of my guides, and that would be that.
But I only want two corners to be rounded. So I draw the rectangle partially off the page, like this:
Here's the trick. I go back up to the Options Bar, and turn my attention to the five little buttons which control how shapes interact:
From left to right, there's the normal shape mode (which I just used to draw my initial shape), the Add mode, Subtract, Intersect and Exclude.
Intersect is the one I want this time:
It's easy! I just draw another rounded rectangle which overlaps the first one from the bottom right corner inwards. Because the tool is now set to "Intersect" mode, I get this result:
You can see where the whole paths are (off the page), but they only create a visible shape where they overlap.
By the way, the layer in the layers panel now looks like this:
You can quite safely leave the shape looking like that, with those overhanging parts. Nobody will ever know they are there except you. But for "cleanliness" of your layout, you might like to tidy it up a bit. There are two ways to do this.
The first way is to press the "Simplify" button in the Options Bar:
However, I don't really advise this. It over-simplifies it, and turns it from a path to a pixel layer, which means you can't re-edit later, or change its size, or anything like that. So, better to go for option 2.
Option 2 is to "combine" the shapes. To do that, you need to choose the Shape Selection Tool (in Photoshop it's called the "Path Selection Tool") from the toolbar:
Once you've chosen that tool (provided you've still got the layer selected) you'll find a "Combine" button in the Options Bar:
When you press it, it gets rid of those extra bits of overhanging paths, and leaves you with a clean simple shape:
(More information about placing images in shapes in this tutorial.)
Needless to say, there are endless practicalities for this method. Once you get used to these five little buttons ...
... you can make all sorts of shapes!
In this one, I started with a rounded-corner rectangle, then "subtracted" the bottom part to end up with another hybrid shape - two round corners at the top, and two square ones at the bottom:
Go nuts! Here I've used a combination of subtraction and intersection to add a title bar:
I hope you have fun with this!