The simplest way would be:
1. Cut out a frame of the desired width from a sturdy cardboard. Make it about 1/2" smaller than the picture.
2. Cut a piece of batting (probably poly for the "fluff") that is slightly smaller than the frame.
3. Trace frame onto back of fabric and cut out about 1" larger all the way around. Cut out the center, also leaving a 1" border.
4. Lightly glue the batting to the frame.
5. Place fabric over the batting and fold to the back of the cardboard frame and glue into place, trimming the corners so that they aren't too lumpy. In the center of the frame, clip the corners close (but not into) the corners. Pull taut and glue onto cardboard.
6. Whip stitch a tab from a soda can onto the fabric near the top so that you have something to hang the picture from.
7. Center the picture in the frame and tape or glue it to the back of the frame.
Enjoy!