Quite simply you cannot. It is widely stated you would need to be at a height of at least 40,000 ft to get even a hint of curvature if earth were round. Commercial aircraft are not allowed to fly this high. They are only allowed to fly just under this altitude. 36,000ft might be typical. In addition, the windows on commercial aircraft are small and heavily curved. Even if they flew high enough for a person to see curvature, it would still not be visible to passengers.
Consider a flat plane. The center of mass of a flat plane is in its center, so the force of gravity will pull anything on the surface toward the middle of the plane. That means that if you stand on the edge of the plane, gravity will be pulling you sideways toward the plane's middle, not straight down like you usually experience when you stand on Earth.

103) There are several constellations which can be seen from far greater distances over the face of the Earth than should be possible if the world were a rotating, revolving, wobbling ball. For instance, Ursa Major, very close to Polaris, can be seen from 90 degrees North latitude (the North Pole) all the way down to 30 degrees South latitude. For this to be possible on a ball-Earth the Southern observers would have to be seeing through hundreds or thousands of miles of bulging Earth to the Northern sky.

59) Quoting Gabrielle Henriet, “The theory of the rotation of the earth may once and for all be definitely disposed of as impracticable by pointing out the following inadvertence. It is said that the rotation takes twenty-four hours and that its speed is uniform, in which case, necessarily, days and nights should have an identical duration of twelve hours each all the year round. The sun should invariably rise in the morning and set in the evening at the same hours, with the result that it would be the equinox every day from the 1st of January to the 31st of December. One should stop and reflect on this before saying that the earth has a movement of rotation. How does the system of gravitation account for the seasonal variations in the lengths of days and nights if the earth rotates at a uniform speed in twenty-four hours!?”

44.) It is in evidence that, if a projectile be fired from a rapidly moving body in an opposite direction to that in which the body is going, it will fall short of the distance at which it would reach the ground if fired in the direction of motion. Now, since the Earth is said to move at the rate of nineteen miles in, a second of time, "from west to east," it would make all the difference imaginable if the gun were fired in an opposite direction. But, as, in practice, there is not the slightest difference, whichever way the thing may be done, we have a forcible overthrow of all fancies relative to the motion of the Earth, and a striking proof that the Earth is not a globe.
Where as some trusting fans believed the post, others were immediately skeptical of the report, perhaps learning their lesson from the huge amount of fake death reports emerging about celebrities over recent months. Some pointed out that the news had not been carried on any major British network, indicating that it was a fake report, as the death of a musician of Rick Davies' stature would be major news across networks.
Besides the above difficulties or incompatibilities, many cases are on record of the sun and moon being eclipsed when both were above the horizon. The sun, the earth, and the moon, not in a straight line, but the earth below the sun and moon–out of the reach or direction of both–and yet a lunar eclipse has occurred! Is it possible that a “shadow” of the earth could be thrown upon the moon, when sun, earth, and moon, were not in the same line? The difficulty has been met by assuming the influence of refraction, as in the following quotations:–
43) If Earth was a ball there are several flights in the Southern hemisphere which would have their quickest, straightest path over the Antarctic continent such as Santiago, Chile to Sydney, Australia. Instead of taking the shortest, quickest route in a straight line over Antarctica, all such flights detour all manner of directions away from Antarctica instead claiming the temperatures too cold for airplane travel! Considering the fact that there are plenty of flights to/from/over Antarctica, and NASA claims to have technology keeping them in conditions far colder (and far hotter) than any experienced on Earth, such an excuse is clearly just an excuse, and these flights aren’t made because they are impossible.
In 2004, the society was resurrected by a man named Daniel Shenton (no relation to Samuel), who created the Flat Earth Society forum, which as of February 2016 has over 8,200 members and 1.4 million posts.[3] In addition, the forum runs a Facebook page with over 14,000 likes,[4] Twitter,[5] Instagram,[6] and Tumblr profiles with several thousand followers each, and a Flickr profile[7] where they advertise a variety of different posters with proofs for why the world is flat. In addition, a variety of independent researchers have attempted to prove that the earth is flat, documenting their work in YouTube videos.[8] The most popular of these videos (shown below) has over 4 million views.
25.) The surveyor's plans in relation to the laying of the first Atlantic Telegraph cable, show that in 1665 miles – from Valentia, Ireland, to St . John's, Newfoundland – the surface of the Atlantic Ocean is a LEVEL surface – not the astronomers' "level," either! The authoritative drawings, published at the time, are a standing evidence of the fact, and form a practical proof that Earth is not a globe.