Design section for each task should cover implementation method for this task - how it should be implemented. For each task in Design section, please include: 
 * Tools that should be used for the implementation
 * What are the specific result requirements
  * What type - code, diagrams, wiki pages, etc.
  * Exact file format and location of the results.
Depending on task type design section should contain
 * For code tasks:
  * Required libraries
  * Implementation methods, suggested algorithms, etc.
  * UML diagrams (class diagrams) where needed
 * For specification tasks
  * Required auxiliary tools
  * Exact text requirements - for example how the user should be reffered (you, You, the user, he, she, ..), What formatting should be used (here you may link an existing page template.)
A good design doesn't leave any questions about implementation - when it's read, implementation should be trivial. Parts of the design may be pointed as implementation.
Use the design section [wiki:PageTemplates/TaskPageTemplate#Design] of the Task Page Template.