How to Add Reflective Mylar to the Windows to Keep Heat In

Measure the dimensions of the glass pane in the window.

Cut the Mylar sheeting to the dimensions of the measured window pane, using scissors.

Apply double-sided tape to the outside edges of the window pane.

Slowly attach the top side of the cut Mylar sheet to the top side of window's double-sided tape. Proceed slowly to ensure no air is trapped, causing bubbles under the film. Place the "shiniest" side of the Mylar sheet so that it's facing in toward the room.

Attach the Mylar to the sides and bottom of the window's taped edges.

Repeat this process for other panes within the window.