Measure and cut two pieces of mat board. Measure the height and width of the picture and use a matting T to measure from the edges of the mat, inward and trace with a pencil to create a precise straight line. Cut along these lines with a matting knife or a sharp utility knife and repeat for the second piece. Take your time so that your measurements and cuts are exact.
Set one board aside (your mounting board) and measure the window for the mat. Subtract 1/4 inch from the dimensions of the picture, then use your matting T again to measure from the edges of the mat, inward and mark this measurement lightly with a pencil. Cut the window out with your matting knife.
Place your picture in the center of the mounting board and hold it in place with photograph corners. Create a hinge with linen tape on the inside edges of the tops of the mounting board and window board to attach them. The taped hinge should be invisible once the frame is on the picture.
Slip the matted picture into a shrink wrap bag, or fold a piece of shrink wrap film over the picture length-wise and make sure it is about twice as large as the matted picture.
Turn on a heat gun and move it over the bottom of the bag or the folded part of the shrink wrap first. Keep the gun moving constantly so that you don't melt the plastic and damage the matted picture inside.
Fold the open top of the bag or the open sides of shrink wrap over slightly to make sure that they seal shut as the plastic shrinks against the picture. Protect your fingers from the heat by holding the folded edges down with a long stick. Turn the heat gun off when the shrink wrap is tight against the matted frame.