No one wants to give anything away?
I'll start. I'm probably one of the rarer examples of someone who did not start out wanting to be a test pilot but ended up as one after all. I started out as an aeronautical engineer who likes to fly. I have only worked for small airplane companies in Europe and the US with small to very small engineering departments and got roped into testing what I designed, since it is always cheaper to have someone on staff do this than hiring a consultant. I'm mostly self-taught on the job, and have learned a lot from flying with other test pilots.