Unfortunately Flat-Earth Theorists - at least on this forum - never seem to explain their point of view.
They never say "I don't understand why the horizon appears flat and so that's why I believe in the Earth being flat".
They never seem to explain their own understanding. They just link to 1-hour or 3-hour You-Tube videos.
It is ironic, because "Flatties" often complain that "Globies" or "Globe-tards" (because you know, play the man and not the ball) never question what they are taught at school and can't explain why the Earth is a globe. Whereas, in actual fact, it is the Flatties that have trouble explaining things. They more often than not just link to a Youtube video.
If a person understands a topic then they should be able to teach/explain it to another person. if two people are sharing a learning experience then they should be able to use Socratic method to arrive at a mutual understanding of the truth.
Let's start simple. Just pick on one thing, something easy, and let's discuss it. The horizon seems like a good place to start as it is something we can all see and should be able to agree on...