First make recruitment manual. In this manual include Job Descriptions (JDs) for all the positions. However, this is not sufficient. Going further, you need to include functional (or technical) and behavioural competencies for each position. Once this job is done then you need to devise various tests, case studies and questions to test each competency. However, sometimes for the same answer to a case study, two recruiters or interviewers may award different different marks because individual judgement may creep in. To avoid this, you need to be clear how much marks to be awarded for each answer.
To do all the above, it will take a long time and it would require involvement of senior persons from each field. While it is easy to decide on the behavioural competencies, it will be difficult to decide on functional competencies. Therefore, if you wish to have quality work, you may better higher external consultants.
23rd October 2016 From India, Bangalore