Test design defines “HOW” something can be tested. It identifies the test cases by elaborating the test conditions identified in test analysis phase or test basis by using the test design techniques which are identifies in the test strategy or test plan.
Test design 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 produce test cases.
As test conditions can be rather vague which can cover a large number of possibilities that can be tested. In test designing we need to be very specific about the test cases that we create i.e we need to have very specific inputs, steps and data for each test case that we create during test designing.