How to use Photoshop Templates:

There will be a background layer filled with Green.

The layer above it is the mat/frame layer. It will have a transparent opening and typically a layer style drop shadow effect. If you turn off the background layer, you should see the transparent opening of the top layer.

Open your image, select all - copy (ctrl/cmd-c)

Go back to frame template.

Select background layer.

Paste (ctrl/cmd-v).

Edit-Transform (ctrl/cmd-t), while holding shift to constrain proportions, size behind opening.