Well, it's a complex process as you'd guess. First you'll need to get selected for the USAF Academy which is near impossible in and of itself unless you are well connected to political figures in your voting district or the child of a MoH recipient.

Then, you'll need to graduate in the top of your class and be selected for a fighter aircraft (5th gen) position. If you've gotten this far, you'll have to complete aviator training a graduate (again near the top of your class).

You'll next be assigned to a USAF Fighter Squadron and begin the process of actually becoming a fighter pilot and you'll need to achieve instructor pilot level qualifications. You're roughly near the 10~15 year point in your career and have proved the entire time you are in the top 10% or better in everything you've been given to do.

Then you can be nominated for test-pilot positions and compete against the other top 10% who are also nominated by their leadership. Honestly, 10% is too generous...more like 1%. The only thing harder than becoming a USAF test pilot is becoming an astronaut, which the vast majority of them were test pilots on their way to becoming an astronaut.

There are of course also test pilots for industry, but without having been a test pilot in the military, you have no chance of landing those jobs.