add 1/2″ seam allowance to all sides of all panels, which equals 1″ total to height and width.
for 13″ w (finished), the center panel may measure 6.5″ w x 13″ h (finished), therefore cut pieces would be plus 1″ seam allowance = 7.5″ w x 14″ h.
side panels (cut 2) would be 1/4″ of the width each.
that would make them 3.25″ w x 13″ h (finished), the cut pieces would be 4.25″ w x 14″ h…. still with me??! almost done with the math portion!
back panel is the exact width of the final pillow case, only add 1/2″ seam allowance to all sides.
14″ w x 14″ h.
back flap is 5″ 