I imported the clip into procreate from the actual episode and edited it frame by frame. I draw a layer on top of each frame that masks the old parts, so its essentially just the original video with sticky notes on top. But to make the additions look more convincing as old cel based animation, i double the linework of my additions, add opacity, and very slightly misalign it with the main linework. That gives it the illusion of the shadows that cel work would cast on the paper