1. Perform a full backup of all User, System and Resource database.
2. Note down of the important SQL Server Configuration Settings, Startup Parameters, Linked Servers, and Script out SQL Server Agent Jobs, Script out SQL Server Logins, Memory Utilization, CPU and Disk Utilization etc.
3. Create an appropriate Service Pack Deployment and Rollback Plan
4. Always install Service Packs first in Development environment and then test all applications which are using SQL Server.
5. Once everything looks good in development environment, then only install Service Pack in QA environment. Test all applications which are using SQL Server in QA environment and also test your rollback plan.
6. If everything look good in both Development and QA environment then plan to install Service Pack in a Production environment by communicating an appropriate downtime window to the stake holders and database/application users.
7. Once the Service Pack is installed successfully in a Production environment perform sanity checks to confirm all applications are working fine.
8. Reboot the SQL Server once Service Pack / Cumulative Updates or a Security Updates are applied successfully on the server.
9. If everything looks goods then release the Production environment for user activities and monitor the environment closely for a week or two to make sure there are no unusual spikes in CPU and Memory utilization.