Read up on traditional 2D tracking methods and how to use markers.
So my plan is to use green screen paper, so when they are holding it at their side the way paper would be after being ripped, it'll still hold true to the video I attach using tracking?
The solution is layers and tracking. You will have to track the left side of the paper and the right side, then attach each side to a copy of the video. I would suggest corner pin tracking in Mocha. If the paper curves when it is torn, which is likely, you'll probably also need to add some distortion effects and maybe even a shadow or two to sell the effect. I probably would not use a green sheet of paper unless the shot was very long because roto on just the fingers wouldn't take very long and a sheet of paper with some a good cross hatch on it, maybe lines about 1 inch apart, would give you more information to use to bend your video to match the bends in the paper. A green sheet of paper, even with some tracking marks on it will only save you the roto time and the shot will look more fake than if you have a grid to work with.
I'd probably use this technique on both the left and right side of the paper: