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Thoroughly clean the inside surface of the windows where mullions will be attached.
Measure the inside area of the window frame and cut the pine parting bead into two vertical pieces and one horizontal piece to fit the space. The finished mullion will look like an “H” with the horizontal piece extending beyond each vertical.
Notch the strips where they cross and attach together with a small tack nail. Paint both sides of mullions with paint in a color to match existing window trim.