It's also in the manual, but there are two ways to do it. One has a part that is fixed with a clip at the back, Another thing is that when attaching the legs, the two frames are engaged with each other.
I came to see how to put the legs on when there are two pieces together and the legs go exactly between those pieces, but you put them on without it facing the camera :( I rewatched though and noticed you set down a silver bracket. I hope I got that piece. I bought these things a year ago but they're still in the box. Time to make it happen or hire someone haha. Edit: I just the order. I have legs, but no specialty brackets. Were those idea pieces or did you have to buy them separately?