Go to the SELECT MENU -> choose SELECT ALL [this will select both the new inner frame and the image for copying].

Go to the EDIT MENU -> choose COPY [this will copy the bordered image]

Go to the FILE MENU -> choose NEW. To create a 100 pixel mat on each side and top and 200 pixels on the bottom, I change the 504 to 704 [as before] but now I change the the 364 to 614 [instead of 564 as above in Method A]. This makes a new photoshop window appear. As before, I now paste the copied image into this window. go to EDIT MENU -> choose PASTE. However, the image will be pasted in dead center. So, now I have to move the pasted image up. To do this I click on the move tool and either drag the image to where I want it or use my arrow keys to move the image. The arrow keys are more accurate. Here's what the move tool looks like.

As before, the next step is to add a color or texture to the white border, if desired. Here's what my new image looks like with a colored, but untextured, mat.

Note that the layers window again has two layers, an image layer and a background layer.