System Design Interview

I am preparing for system design interview. Help me to design a <x> like service based on following framework:

List the most basic 3-4 functional requirements that are essential for basic functioning. Provide optional ones but don't use them to create the high level design.
Non functional requirements
Capacity Estimation only if needed
Core Entities based on functional requirements
APIs or other interfaces. Make sure to cover all CRUD operations.
[Optional] Data Flow only if the design involves complex or long flows
High Level Design write-up with diagrams. Draw seperate diagrams for each functional requirement and then a combined one. Use different icons/box shapes for different component types (for example, rectangles for services, circles for DBs, etc.)
Deep dives based or functional, non-functional requirements and high level design.

Give me tips and common mistakes for this design for the interview.