This is the guitar part for bass.  There is no actual bass, JPJ uses footpedals and plays a keyboard for it.

Okay, play this only once the first time at the beginning of each verse

G-222-
D-000-

Then, do this (main riff)

G-222-----333-333-----444-444-----555-555------777-
D-000--0--000-000--0--000-000--0--000-000---0--000-
A--------------------------------------------------
E--------------------------------------------------
When the music changes (chorus), do this

G-12-11-10-9--7-6-5-4-3-2------
D-12-12-10-10-7-7-5-5-3-3--320-
A------------------------------
E------------------------------

Later on when you go to the bridge, you end with

G----------
D--320--0--
A-----02---
E----------

Instead of

G-----
D-320-
A-----
E-----

During the bridge(1) you play this

G--------------------------------------------------------------
D----------------7/(0)------------------7/(0)-------320--0-----
A--0---0---0-----------0----0--0--0-----------0--------02------
E--------------------------------------------------------------

Hold the A (7) before you slide

On the fadeout and the second bridge you play

E--3--5--

on and off (i usually play a along with the drums, because it's hard to hear the exact rhythm). You should be able to hear the change, and if you can't listen for Bonham to hit the cymbals

Thanks to ToOlBaSsIsT for your corrections.