It's actually pretty easy to do simple formation in DCS. Just go into the mission editor and set up whatever planes to fly a course and start your player airplane with them. When you launch the mission you'll be near them and can then saddle up close.
But all you can do are simple straight courses and turns. No loops or rolls and the more complex stuff. And if you get too close to them they will reposition away from you. And they have to be on the same side as you or else they come at you guns and rockets blazing.
I haven't tried in the new version but it worked well in the old version and I have some videos up from when I was flying DCS before DCOC or FlyInside.
I need to get back in the groove with DCS though!