Hi Cathy -
there are a few companys that sell card cut to size already - CM do them in a pack with triangles and stripes as well, but generally I cut my own as I usually crop the photo first.
What I do is stick the photo to the card, then I trim it - I use a CM personal trimmer for this, and I find if I line the photo up with the plastic guard on the trimmer then that makes a nice size mat - approx 1/8th inch,
I also quite like the coluzzle - I use it for circles and ovals (I do have a square one too, but I don't use that very often)
That makes 1/4 inch mats - you cut the photo on one line and the mat on the next - I think 1/4 in is a bit too big, so I have a companion oval template, which means you can make 1/8 in mats - the only thing I would say against the coluzzle is that I am not convinced the ovals on the standard and the companion are exactly the same shape - they seem slightly thinner on one.
Yet another tool - which I use mainly for matting letters, is the magic matter - this is a disk (you get 4 of various sizes), you put a pencil in it, and run it around the edge of your picture, letter etc, and it draws a line exactly 1/8 in out - then you can cut along the line using scissors.
Hope that helps,
Bev