How can you be an atheist and believe in a flat earth? That boggles me.
Simply by not having faith. Not even enough faith to believe the scientific consensus. I used to have so little faith that I couldn't even believe that airplanes exist, even though one of my friends claimed to have been in one.
But now I think there is a better way to live: have faith in those for whom you have good reasons to trust.
You have good reasons to trust science: it gave you medicine, it gave you a computer, it gave all that fun you had in school learning it…
And what did religion give us to trust it? I am not going to refer to any specific religion here. Circumcision, FGM, human sacrifices, animal sacrifices, inquisition, indulgence, holding back the science, crusades…