Flight testing is a vast subject so it depends on how committed you are. I started flight testing simply by gaining a flight test approval from the CAA to perform airworthiness flight tests and C of A renewals on already certified aircraft. For this I had to have a breifing from Darrol Stinton, the then CAA flight tester, way back when. Darrol was an extrodinary man and taught me more about aircraft over the years than I can remember. After a while I built up a reputaion with him and the CAA such that He would invite me to flight test certain aircraft for the issue of a British C of A and to evealuate some vary strange machines, often before he had the chance to fly them. I also got involved with accident and icident flight testing to try to find answers to some accidents etc. Prototype flight testing, however, is mostly done by pilots who have gained some real qualifications in the subject and are mostly employed by the manufactuers themselves. So if you just fancy just the mundane stuff and you have the experience, ask the CAA what the current requirements are and then get along to your local light aircraft maintanence company and see if they might put you forward for flight test approval. Then build up a reputation.