Measure the width and height of the door to be replaced with the tape measure. Round up these measurements to full inches. For example, if the width of the door is 35 3/4 inches, round to 36 inches; do the same with height. Write down measurements.
Measure the jamb from the rear of interior trim to the rear of exterior trim. This measurement assures a flush fit. Write down the measurements.
Install the steel door per factory instructions. Most steel doors are pre-hung to ease installation.
Take the measurements along when deciding to order. Steel doors can be ordered from many hardware stores.