We're working to get this area of the documentation cleaned up. At this point, it's honestly quite fragmented and difficult to apply to specific customer configurations.
Given the basics of your config, I'd think you'd want to set it up by modifying the Advanced Properties>Web Services section of the /immanage console for your IM environment to use SiteMinder Other Authentication and have that refer to your custom auth module.
With this sort of complexity, you may be better off opening a support ticket for more specific assistance and guidance.
W.
William Lee
CA Technical Support