Plan a single iron entry door by confirming the rough opening and clearances end-to-end.
Width & height: Measure the rough opening in three places (top/middle/bottom and left/center/right) and use the smallest numbers. Note threshold height.
Plumb, level & square: Check jambs and header; record any out-of-level or out-of-square so the frame can be sized accordingly.
Jamb/wall depth: Measure wall thickness to match the frame profile and interior/exterior trim.
Swing/handing: Decide inswing or outswing and left-hand or right-hand based on approach, stair walls, and hardware clearance.
Surrounding space: Confirm room for hinges, pull/handle projection, and door sweep; verify space for sidelights or a transom if you plan them.
Obstructions: Note light switches, receptacles, casing, and flooring transitions that affect frame size and threshold choice.
With these dimensions set, selecting a single iron door (modern, arched, or French) and the right glass/finish becomes straightforward.