Measure the window pane with your tape measure. Cut a piece of cardboard using the dimensions of the pane. It can act as a stencil for your Mylar sheets.
Lay the Mylar sheet over the cardboard. Cut it out in the shape of the cardboard.
Place one piece of double-sided tape on the top of the pane and press it down firmly. Apply another piece to the right side, another to the left side and one to the bottom.
Place the bottom left corner of the Mylar you cut against the bottom left corner of the glass pane, holding the rest of the Mylar away from the tape. The mirrored side of the Mylar should be facing you. Press firmly on the Mylar to attach it to the tape across the bottom of the window.
Run your fingers up the Mylar, over the tape on either side of the window. Keep it taut so it covers the entire surface. Press to attach it to the tape at the top of the window. Continue the process until every pane has Mylar over it.