I got that part and most of it from another person, though I added a bit here and there.
So this part has been a bit confusing for me as well, but I think that once you have done your
'perceived discovery of external error' by dropping metal balls from where the author's claim doesn't match your observation,
you will need to list all the things that you think are relevant to what led up to your discovery.

Now I stole the above image from wikipedia, but it's stuff like that that I assume you should have a gallery of,
so that everyone and your grandmother knows what we're talking about and don't mistake it for anything else.

So one's list (the hypothesis) should at least consist of

1. The leaning tower of Pisa (A nice little picture, where it's located)
2. A big metal ball (what it's made of, where did you get it)
3. A small metal ball
4. Planet Earth
5. The air (and why you think that's relevant)
6. The dropping mechanism (I'm assuming one's hands)
7. The exact section (book, page, paragraph) where it says that they should be falling at different speeds
8. The above image showcasing what and a video of you dropping the balls

And that's for the observation that lead to the perceived discovery of external error.
Then you will need to add to the list of what your experiments need.
You know, a stopwatch, more objects, 3D models of those objects,
a better dropping mechanism and a 3D model of that so that people can recreate your experiment,
an air chamber, where you can increase and decrease the pressure.
Stuff like that.