• summarize the system requirements   (cf. Background)

• describe the design/implementation   (what exactly did you do?)

• justify your choice of methods   (why did you do it this way?)
  • platform, architecture, algorithms, data structures, ...

• outline alternatives that were rejected

In all of the above, follow an established software engineering methodology.

For an interactive system, adding a tour of the interface is very useful to give readers a feeling for what you've done.