1. Choose location, with possible consequence to choice of best ladder. A hallway will have least consequence of cut joists, with easiest compensation. Think more of safety and comfort of egress in the attic, than of easy approach at bottom. Symmetry of placement in a ceiling usually matters less than freedom to get around a deployed ladder in a hallway. Be very willing to not use the location of an existing opening.
2. Choose the ladder. Within living spaces, choose a ladder with a insulated and gasketed door. In a garage without fire separation of attic spaces, choose a fire-rated ladder. A wood ladder has best customization flexibility. Plan the ladder deployment to clear obstacles, and to reach the floor with least-steep angle. Apply finish in multiple coats to a wood ladder.
3. Clear obstacles, and prepare the rough-opening frame. Within living spaces, use tall framing matched to the depth of required insulation, e.g 2x10 minimum for R38. Expect that 2x4 as with truss bottom elements, is insufficient to carry storage loads. Never cut a truss. Follow local building codes.
4. Decide location in elevation, of the ladder frame vs. joists or rough opening frame, and the ceiling surface. With ceiling imperfections, think to have the ladder door face slightly recessed, with its own sense of horizontal. Don't hide alignment error and gaps about the ladder frame with decorative trim; where as usual, the installation must be airtight.
5. Cut the opening. Use dust barriers if needed. Protect floors with blankets. Plan cuts and temporary supports to avoid dropping materials and break-away into surrounding ceiling area. A well-set rough opening frame serves as a saw guide. Ensure the ladder frame will not load and break edges upon insertion, but avoid oversize opening. In living spaces, or a garage, the ceiling must be finished air-tight, fire-tight.
6. Remove the ladder step assemblies, probably as a unit. Steps are unstable weight and obstruct access to screw the ladder frame to the rough opening.
7. Install the door and frame assembly or just the frame, to the rough opening, working with shims, and if possible, with adjustment freedoms left in the RO frame, to minimize gaps outboard of the ladder frame. The door must work squarely within the ladder frame, as screws are finally set. Set the door angle adjuster (option) to least-steep angle consistent with a strong bottom step upon trimming.
8. Install the steps assembly at highest position, up from the door top face and closest to the top, hinge-end, of the door. A lower position on the door, if an option, may be set, if needed to reach the floor at chosen door angle and favorable bottom step location.
9. With the bottom steps assembly removed or folded-back, measure the distance to the floor from the leading-edge bottom of the next-higher section. If greater than the length available in the bottom steps section, choose a steeper angle setting. If there was a planning error, and the received steps assembly (with three sections) can't reach the floor, use a kit or other resources to customize the ladder with a fourth section.
10. Mark the bottom leading edge of the bottom section, and sloped cuts relative to that, by template or by angle measurement. Test, cut and fit. Smooth the cuts.
11. Attach leveler legs, if employed.