This is not “programming” related as much as it is “software engineering” related.
I am required to produce an implementation for additional functionality to a complete system. All I am armed with is knowledge of the existing architecture and a functional spec with visual requirements, user stories and use cases. Is there a standardised way to go about this? I suck at documentation.
Analyzing and designing a solution often follows a process a.k.a. methodology. The outcome of the steps are artifacts a.k.a. documents.
This related post indicates some free methodologies:
Rational Unified Process Alternative for Documentation and Templates?