Final answer:
The incorrect guideline for preparing a Data Flow Diagram is 'Give each process a sequential number.' DFDs represent data flow, not process sequences, and processes should be identified by their function, not by a numerical sequence.
Step-by-step explanation:
The guideline that is not recommended when preparing a Data Flow Diagram (DFD) is "C. Give each process a sequential number." Data Flow Diagrams are used to visually represent the flow of data in an information system and assigning sequential numbers to processes is not a standard practice as it implies a sequence which may not exist. Instead, each process is typically given a unique identifier that relates to its function within the system.
DFDs should be subdivided (Subdivide the DFD), transformations should be grouped (Group transformation processes), and no aspects of the system should be ignored (Do not ignore any aspects of the system) as part of the guidelines for creating an effective DFD.