Test design defines “HOW” something can be tested.
It identifies the test cases by elaborating the test conditions identified in test analysis phase or from test basis.
As a tester you need to use test design techniques which are identified in the test strategy or test plan.
Designing can start once the test conditions are identified with sufficient data to produce high level or low level test cases.
For a given test level, test design can start once the test conditions are identified and enough information is available to write test cases.
As test conditions can be rather vague which can cover a large number of possibilities that can be tested.
During test designing we need to be very specific about the test cases that we create.
We need to have very specific inputs, steps and data for each test case that we create during test designing.
Test designing requires detailed analysis and understanding of requirements.
Test Analysts need to first apply static testing techniques like reviews, walkthroughs to understand the requirements in detail.
Once the requirements are clear, you need to apply test design techniques like Equivalence Partitioning (EP), Boundary Value Analysis (BVA), Decision Table Testing, State Transition Diagrams to write your test cases.