hello!,now we are going to make your seed in you flat world,there are 6 steps.
1.click "Create New World",then named your world.
2.next,click gamemode in 2 times then stop in Creative.
3.then click to "More World Options",then click "world type:superflat".
4.click to costumize then click presets.
5.then copy the seed.
6.then click use preset.