Check both door jambs for vertical alignment using a large level. Since the Designer Door system is a combination of door and frame you need to ensure all measurements are taken from the doorway surround i.e. from the outer extremities of the existing door frame.
Carefully measure the vertical dimension of door aperture between door lintel and sill at three positions across the doorway (along hinge edge, at door midpoint and along door opening edge) ensuring the measure remains vertical when taking each reading. The shortest of these three dimensions will be your door frame height
Carefully measure the horizontal dimension of door aperture between door jambs at three positions between lintel and door sill (top of doorway, door midpoint and door base) ensuring the measure remains horizontal when taking each reading. The shortest of these three dimensions will be your door frame width
Proceed to the Request a Quote page
and follow the instructions there.