Open the existing attic door and place wood shims under it, propping it up as you remove the hinges.
Pop the pins out of the hinges using a flathead screwdriver to pry them up and out. You may have to tap on the screwdriver with a hammer to get the pins to come up and grasp them with pliers to pull the pins from the hinges.
Pull the old attic door off the hinges and set it out of the way. Unfasten the screws holding the hinges in the attic entryway using a screw gun. Leave the wood shims where they are so you don't have to measure the gap between the floor and the bottom of the new attic door.
Set the new attic door on the shims, balancing it in place and against the attic entryway.
Align the holes in the hinges with the holes in the attic entryway and fasten the hinges to the doorway with screws and a screw gun. Then fasten the hinges to the new attic door with more screws using the screw gun.
Pull the wood shims out from under the attic door, allowing it to hang on the hinges. Swing the new attic door back and forth to ensure it opens and closes.