Steps to install service packs on active/active cluster

Hi All

We have Install the Service Pack 3 on SQl server 2005, can you please let me know Installation Steps.

Below Environment we have.
We have Active/Active Cluster Environment on SQL Server 2005 SP 2, Windows 2003 Enterprise Edition.

It would be helpfull if i get the steps to be taken Before,during and after installation of Service packs.

please suggest.