Create a frame inside the frame...

Mat (or Passe partout) is the term used to describe the "Window-cut" material placed around an image within a frame.  Mats add both an aesthetic element and serve a functional purpose: they serve as a spacer and protect artwork and photographs from direct contact with the Glass.  From an aesthetic point of view, the mats provide several design qualities that enhance your artwork: they make the overall size of the finished piece larger and provides a space for the eyes to rest between the art and the frame. They provide depth, width and color to your framed art.

How to cut a regular picture frame mat
Decide how much of a mat Border you want to leave around the subject. As a general rule, leave at least 3 inches on the sides and top and 4 inches on the bottom.
Calculate how much art you want to show through the window.
Cut a piece of mat board to fit your picture frame. Measure the interior of the frame and mark these dimensions on the back of your mat.
Place the mat on a cutting board or cutting mat.
Place a straightedge along one of the lines, hold it steady, and run a cutter along its edge.
Measure and mark the back of the mat, drawing the window that you'll cut out.
Cut the window from the back using your mat cutter. Place the straightedge along one of the lines, hold it steady and run the cutter down the edge.
Stop right at the window's corner so you don't overshoot.

Cutting a Double mat
you start cutting a double mat the same as a single mat, making that first window ("top mat")
then cut the "bottom mat" board 1 inch smaller than the "top" mat. You need to use the same outside edge to cut both openings.
use double-sided tape inside the window cut-out to secure the two boards together, in position.

The main things to remember:
measure twice, cut once
change the blade of your cutter often