Lift the truck and set jackstands under the frame so the rear axle is hanging on the springs.

Remove the wheels
Remove the shock from its lower mount
Unbolt and remove your ubolts on one side
Use a floor jack to lift that side of the axle so you can get to the centering pin
unbolt the centering pin
install your AAL and centering pin again.
lower axle on the pack making sure the centering pin finds its hole.
Redo the Ubolts - tighten in a criss cross pattern and crank 'em down.
Repeat on the other side.

Is this a single AAL, or a 3 pack kit? Not sure about the 2nd gens, but the single AAL is responsible for a lot of broken leaves on the 1st gen Xterras and Frontiers.