If you're asking the speed of the Sun as described by modern,

**real**, science, then what you ask is a fairly easy task. In short, parallax measurements are taken to determine the distance of the Sun from the Earth. From this, the orbit of Earth can be plotted. Conservation of energy and momentum gives the speed at various points in the orbit. If you want a detailed description, see

https://en.wikipedia.org/wiki/Orbital_speedAs for what it is in Flat Earth, well I'd assume it's just taking the radius of the Sun's circling and dividing by 24 hours.

Parallax is clueless about even elementary physics when he says "speed and distance are not the same thing." I think most anyone can understand that if we can measure a distance and know the angles at which we measured that distance, what we have is a coordinate system that uniquely represents every point in 3D space. If we know the position of a body at every point in time, then we can obviously differentiate that to get the velocity. Taking the magnitude of the velocity gives the speed. In layman's terms: I see the car at point A at t=0. I see the car at point B at t=0.001s. Therefore its speed is dist(A, B)/0.001s.