Also known as the Newberry Method, although there's some question as to whether or not that's completely accurate. Quick and dirty instructions:
1. Cut a piece of acid free foam board an inch or two larger than the frame size.
2. Lay out an opening in its center a bit larger than the mat window opening.
3. Cut out the opening with a straight, not beveled, cut.
4. Place the needlework on the fallout, center and smooth it out.
5. Replace the foam board "mat" onto the fallout, catching the fabric between the two pieces of board.
6. Pull and tuck the extra fabric from the back to get it taut and smooth.
7. Remeasure, cut the mats and place on the needlework. Trim excess foam board.
Depending on the thickness of the fabric, you may need to trim the fallout a little bit, but the fit still needs to be a bit snug.
More complete instructions, complete with pictures, are available in the PPFA Guidelines to Framing Textiles and Needlework.