Hey, thanks for the comment! I hope I understand those questions correctly.
How do you do this?
Magic Sufficiently advanced technology.
Well, after I had the idea, I started with the sheet of paper (one of the first frames on the top of the image). I used the airbrush tool with a small size to make the sketch. Then, I went from there and sketched the sheet over and over again, moving a bit from the last version. Towards the end, I sketched the stack so that I knew where the sheet would end up. Then, it was only a matter of moving the sheet fall down to the right position and trying to adjust its velocity and the perspective (in most of the animation, I didn't bother with perspective, which is great because it doesn't really matter in that case). And then, I had to sketch the sheet falling... upwards, so that I could have the sheet fall off frame down into the frame later on.
Finally, I traced the sketches with the brush tool (with about twice the size as the airbrush and smooth stroke settings). As a last touch, I added the blue shading.
Oh yeah, and ordering the frames correctly and checking what speed it should be displayed with and optimising...
What kind of app?
I use the GIMP as a desktop program. I've never had the opportunity to use it as a mobile app.
I hope this answers your questions. If not, I'm afraid you'll have to be a bit more specific.