This is an easy tutorial that makes a very simple border for banners and such.

1. Use the Rounded Rectangle Tool over the image you want the border on. It should look like this:

2. Flatten the image and use the Paint Bucket Tool and use a different color than the one you used for the first step. It should look like this:

3. Erase the middle with the Magic Eraser Tool. It should look like this:

4. Go to Layer Styles>Wow Plastic and choose a color that best fits your banner.

Here is my final product: