Determine your situation. Most installations of an attic ladder are a replacement of an existing ladder. This instruction set focuses on replacement.
Inspect opening ensuring all fasteners and previous blocking are removed. Make sure the opening meets a minimum structural requirement of a 2x6 header and joist system with a rough opening larger than the attic stairs.
It is good practice to add two extra 3" deck screws or 16d nails to each side of the headers (short sides of the opening) for reinforcement.
Attach temporary support boards at least 4 inches wider than opening using a drill and 3" deck screws. Fasten the boards to a dimensional spread to hold the frame of the new ladder on the short side without impeding the ability to open the door. The temporary supports need to be placed centered with respect to the long dimension of the opening. They will hold the stair in place while it is fastened to the joists and headers.
If the ladder is more than 1-1/2" smaller in any dimension than the rough opening, secure additional framing in increments of 2x4s (1-1/2" thick) and 1x4s (3/4" thick) with 3" deck screws or 16d nails at 12" on center spacing. The goal is to get less than a 3/4" gap to fill with shims.
Install the pull rope on the ladder assembly by threading the rope through the hole in the door. Then, thread the washer (included in the attic ladder kit) onto the rope on the upper side of the door. Pull the rope through to your desired length so it is long enough to reach from the floor, but not too long to walk underneath when the ladder is in the closed position. Tie a knot on the upper side of the washer. The washer prevents the knot from slipping back through the hole when the rope is pulled.
Leading with the hinge side of the ladder, lift the ladder assembly up into the opening using the temporary support boards as a guide. Ensure the hinge side is on the correct end. You will need two people for this step. One person should be in the attic, and one person should be on the lower floor.
Once ladder is positioned correctly, pre-drill and insert four 1/4" x 3" hex lag bolts into the header side (Image 1). If needed, add additional shims along the midline of the long sides of the ladder (Image 2).
If the ladder legs are too long, fold the ladder at the lowest joint and measure from the hinge to the floor (Image 1). Unfold the lowest section of the ladder, and cut the straight legs just enough for the feet to sit correctly on the floor. Use a hacksaw if the ladder is metal, and a wood saw if the ladder is wood (Images 2 and 3). Re-attach the feet using wrenches (Image 4).