print aspect ratio after adding a frame to photo

I'm finally getting comfortabe using PS7 to modify and enhance my photos - I'm now ready to start printing my work. I have no problem printing 4X6 unchanged photos directly from the camera - sometimes there's a little cropping, but for the most part they look fine.

My problem starts when I try to print photos to which I've added a mat and frame. I don't know how to maintain the appropriate aspect ratio for printing to 4x6, 5x7 and 8x10 size formats. (I know what the appropriate ratios are, e.g. 1 : 1.5, I just don't know how to transform the photos to make them "fit".) I suspect that in the process of creating the mat and frame I should be taking their measurements into consideration, but I'm not sure even how to begin doing that... If anyone can point me in the right direction, or to a detailed tutorial on this topic, I'd greatly appreciate it.

I apologize in advance if my "lingo" isn't correct. I still have lots to learn!

This is a link to one of the photos I want to print, just in case I didn't explain myself properly.

<http://www.walmartphotocentre.ca/walmartcanada/control/photosite/dtomei>

Thanks,

Dominique