1) Pre-SLA. Before you announce the SLA, practice as if you have an SLA. This includes the following:
a. training the Support Center staff,
b. monitoring response time to ensure SLA compliance,
c. resolving cases in accordance with the SLA,
d. setting customer expectations,
e. managing the work flow within the Support Center and between the second level.
2) Training the rest of the department.
a) Weekly meetings. For the first month it is beneficial to meet regularly to find ways to improve communications and workflow, review Cycle Time Reports and exception reports.
b) Service Group Feedback. Ask other service groups to review their cases for the proper Severity Levels. If they find discrepancies, ask them to provide a list of case types and the appropriate Severity Levels. This will be the basis for Support Center training and documentation.
Here is a great White Paper on Implementing Service Level Agreements. See https://www.givainc.com/wp/it-help-desk-software-cloud-whitepapers.cfm