You must understand the requirements for your product, and analyze how these requirements broadly map to the Engine's decision details concepts before starting implementation.
Unlike the requirements for eligibility/entitlement, typically requirements for decision details are not enshrined in legislation, but instead are based off informed predictions of the kinds of information that may be useful to a case worker, when the case worker is trying to understand a complex case (and perhaps answer questions about the case from customers.)
As such, producing requirements for your decision details is perhaps more art than science (in a similar way as for key decision factors; however, requirements for decision details tend to be more complex than those for key decision factors, given the flexibility of the Engine's support for decision details and the wealth of data items that are candidates for display).
For a new product, you might consider revisiting your decision details requirements once a product has been live for some time, in light with the kinds of information that case workers are attempting to understand when they view determinations. If you layer your requirements and implementation according to the recommendations, you should be able to implement and deploy changes to your decision details rules without affecting any underlying eligibility/entitlement calculations.
The following steps should aid your analysis.