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Before attempting to install the door, it's crucial to analyze the rough opening to ensure it's suitable. First, measure the width of the existing door at the header and the height of the frame. Then, measure the rough opening itself. Ideally, you want to have a 1/2 inch space for width to allow for adjustments to achieve perfect plumb, and a 1-inch space for height to accommodate future flooring and allow the door to be lifted. Next, use a 6-foot level (or a 4-foot level if a 6-foot is unavailable) to check the plumb of both sides of the rough opening in both directions. If the framing is perfectly plumb, you can maintain tighter tolerances. If it's significantly out of plumb, you may need to adjust the framing using tools like a sledgehammer or sawzall, adding shims or cutting plates to achieve the desired space and plumbness. Finally, check how level the floor is across the opening using a 2-foot level or a laser level. If new flooring is being installed, shim the door up with temporary shims to the exact thickness of the new flooring on the low side, then cut the jamb with an undercut saw once the new flooring is in. If installing over existing flooring, you might need to cut down the door frame accordingly based on how off-level the floor is.