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Begin by measuring the precise width and length of your attic ladder unit. In the hallway or designated installation area, find and mark the center of the ceiling. From this center line, measure out half the width of the attic ladder to mark where one edge of the opening will be. Repeat this process on the opposite end of the hallway to establish the second parallel line. Use a level and pencil to connect these marks, creating a clean, straight line. Repeat for the other side to fully define the width of the new opening. To determine the length, locate an existing joist in the ceiling and use it as a reference point. Measure back from this joist to mark one end of the opening. Draw this line, then measure the full length of the attic ladder from this first end line to mark the opposing end. Again, use a level to ensure all lines are straight and the rectangle is square.