Dean
Your post is a little ambiguous
If you want to get in to ETPS you get a pass from the main gate and walk in through the door.
But if you mean that you would like to become a student on the course you apply as per the current RAF orders (sorry I don’t know what they are called today but your squadron admin people should be able to help you)
However, if you want to join the staff of ETPS you will have to do quite a lot first either in the field of flying or education.
Test flying should be seen as a vocation not a job.
Now a question for you – and this is about the hardest one you will have to answer in any job interview – why do you want to be an xyz? (in this case test pilot). It was the first one the Commandant of ETPS asked me in 1954.
Not trying to put you off, but “would like” is unlikely to be enough. Getting the training you crave at the expense of anything else in life would be a better starting point.