Guide to start the opportunity /lead DCM from scratch, without current OOTB DCM's being in the way

Even though it's great to have an opportunity DCM best practice example, we encounter the need for most clients / prospects to review it from scratch.

However, the OOTB Opportunity and lead DCM's are quite heavily embedded in the system, and it would be great to have a step by step guide to explain which processes are active and linked to these 2 DCM's and how to deactivate them.

Also, how to handle the current entries (lookup data for example ) so to make sure that they do not appear in filters etc (as it is not recommended to delete OOTB fields).