Knowing where to start is hard to say without knowing what you have currently already built in that area.
For example, have you already deployed horizontal discovery and now you are looking to take the next step with Service Mapping, or do you need to stand it all up from scratch. The HLD will probably require a lot more input if you have not even begun any discovery to this point, as you will have a lot more work to do regading credentials, MID server locations, integrations etc. If you have already done that part, then you are probably looking more at how the various mapping capabilities work and how to enable them, designing the flows to keep your service map current, persona mapping etc.
