I'm convinced the idea of a flat earth is mostly the ultimate troll job by people who don't actually believe the earth is flat to try to convince others that it is... as a joke!
Nevertheless, trying to come up with a possible explanation of how modern scientific evidence could have an alternative theory that still operates under a flat earth is quite the difficult puzzle.... Why? Probably.. I don't know, because the earth actually is spherical?
Flat earthers either think the earth is a circular pancake-like disc moving up creating gravity or that density is the only reason for gravity. If you ask another group of flat earther's of course, that's just a conspiracy put out by "round earth theorists" trying to make them look bad... mostly because the "mainstream" flat earth theories have been thoroughly discredited already. In the flat earth world, antartica is the edge of the world and rather being south of every continent, it surrounds the outer wall of the earth's edge and prevents the oceans from flooding off the edge I guess.
Well trying to prove a round earth theory to the flat earthers is actually not as easy as you might think. For example, if you say "well if you stand on a mountain top you can see things farther away because you have an angle to see over the horizon line" they might try to tell you that vision improves with higher altitude or something.. So It's the ultimate trolljob because they seem to have an answer for everything. Not necessarily always a good one.
Also, they often don't accept your evidence or have another explanation for it which may contridict a lot of other basic things and require a whole new set of beliefs in a different area. As such, they disagree with one thing and you have to explain to them something else and they debate you on that.
So here's how you can prove a flat earther he's wrong. Before they change their interpretation subjectively to fit their beliefs, you have to agree on what will constitute as proof. Say, okay, if this map you're showing me is correct, the distance from Chile or Argentina to China would be less than India or Australia. If the earth is spherical, you could go underneath the bottom of the earth at enough of an angle to take a huge short cut while avoiding the harsh weather conditions of the Artic, right? If they don't understand show them a globe.
So you set up a bet and say if they will accept the flight arrival times are accurate (something that may be difficult to do because it challenges their beliefs) that you can prove to them that the earth is flat because the flight arrival times are shorter. If they won't accept that, make them prove it by taking a flight and if the flight lands and they have proven the times match a flat earth through video evidence of flight landing and looking at the local clock, you'll pay for their flight. If you're right, they have to buy a flight.
Of course, if they try to argue that the plane goes 3 times faster when it goes farther away, you know you can't actually convince them of anything. But if they can come up with some elaborate hypothesis of the earth being bowl shaped and the spin of the earth keeping things on the sides to stick without blocking up the sky or something, then good for them, you can challenge them to come up with a way to test the hypothesis.
Well, here's the tough part. When I tried to look up a video debunking the flat earth by showing the flight times, instead I get some flat earther who says those flights don't actually exist. Well I was done at this point, but at some point if I'm super bored I plan on actually proving those flights exist, not by actually going on them because I think flat earthers bare the burden of proof, but by finding aflight that lands at a particular time. You also don't necessarily need to start with hand picked locations that should be the most obvious. You might instead look at somewhere close to you where you can actually see people getting off a plane at the airport if you wanted to after tracking it's flight arrival times. But that's absurd amount of effort to exert for a bunch of trolls that once you get back from a flight around the world probably would respond... "haha, I tricked you, I knew the earth was round all along! You Got Troll'd!"

