1. Backup of all the System + User Database
2.Windows Login account with "Local Administrator" privileges on all the nodes involved
3. Ensure all the recourses part of the SQL Server Cluster resource group are online.
I recommend to reboot both nodes before installation. (to avoid pending reboot, hanging installers, terminal sessions etc. problems.)
4. Cross check if the cluster failover on the nodes involved successfully.
5. shut down all applications that use SQLServer (because you know the instance(s) will get offline at some point(s) during the upgrade)
6. Disable if any DDL Triggers
7. Verify if following services online on the nodes involved.
- Task Scheduler
- Com+ System Application Service
- Cryptographic Services
- Remote Registry Service
8. Pause Database Mirroring
Log on the active node, apply the Service pack. ( It is opposite in case of SQL Server 2008 ! ). You cannot install SP3 from any passive nodes in the failover cluster.
Please note SP will not update the client tools on the passive node. you need to do that manually; though it is cluster aware.