Here are some tips I can provide you from when I learnt my first steps in formation flying.
Start off easy. Create a small mission with an AI as lead, in your case, a P-51 with a good stable cruising speed(about 250mph). Fly your P-51 close to it and hold a decent distance. The positioning doesn't matter at first. It is important that you fly close to it and learn to use the throttle in formation.
Once you feel comfortable enough with the throttling you should focus on the positioning, still only in level flight without any turns. Join the leading AI in the 'Slot' position to learn to use the elevator controls to stay in position. Don't use your rudder while learning the first steps. They come in use later.
Once you feel confident behind the leading aircraft, change your position to either side and make yourself more familiar flying a solid and somewhat stable position. Once you feel good in level flight, you can go for your first turns.
To practice turns, you may look for a partner to practice with. Someone who can keep the aircraft very stable as leader and who has experience(sounds easier than it is, I know...).
Following an aircraft through turns is not far away from flying your first rolls in formation. It needs more time to practice to work with your throttle and the stick simultaneously. Again, don't use your rudder yet but control your distance to the leading aircraft with your elevators by carefully banking in and out.
For the advanced stuff that goes beyond military purpose, you need an experienced leader to practice rolls and later, loops.
Depending on how much time you can put in practicing formation flying, it can take you up to 9 - 12 month to be good at it. Some are faster, some need more time(I was one of the later).
Joining a virtual air demo team with already existing skills and experience in formation certainly advantages you to join for live shows sooner, so that you won't need to learn the basics before flying the real stuff. Although, many teams welcome everyone who have never flown formations and/or aerobatics before because the amount of virtual airshow pilots is only a very small portion in this community.
Good luck and happy flying! :)