In the flat earth model, the sun remains constantly above the earth. This is easy to understand. It has to so that it's always daytime somewhere.
So take a piece of paper, and draw a line on the paper. Make it long. Mark a point on the line, and put a little notch above the line to indicate the observer. Okay? Okay.
Now draw how a sunset works in your model. Basically, at some point, the bottom of the sun MUST be below the horizon, and the top of the sun must be above the horizon. However, the sun must also be completely above the horizon for another observer on the plane. So please indicate a point on that line where it's possible.
It's trivial to show how that works on a sphere.
As your second exercise, I need you to grab three world cities. Make them far apart, just to make everything easier for you. Now reduce this to a scale that fits on your paper. 1 cm per 500 km might work, 1000 kms for a larger distance. Whatever works for you, so long as you make it consistent.
Then independently find out the distances between those three cities, and make a triangle between them using your decided scale. So for example, if you decide on cities that are 1000, 1500, and 2000 km apart, and you decided on a 100 km/1 cm scale, then you'd make a triangle with sides of 10cm, 15cm, and 20cm. Just so long as it all fits on your paper.
You can do this with any three cities.
Here's the catch: as soon as you add a fourth it all falls apart. You can make a triangle between the first and second and fourth cities, but that fourth city and the third will never have the right distance. It will NEVER work, unless you fold the paper and use three dimensional space.
Strangely enough, this works without fail on any decently made globe, provided you use the same scale as the globe.