1. download pocketinveditor pro
2. while download or before downloading make a creative world in mcpe , at spawn dig down and when you hit the last bedrock , exit mcpe
3. when its done downloading > open the app > click the map that you want to be flatten > go to edit terrain > then click user player coordinates > in width (x):500 height (y):1000 length (z): 500 > click set block > click browse > select the first one and then click ok >wait while reading terrain data is analyzing and when its done go back to mcpe > open the world > see if it works

p.s.
you can do it too in the surface of the map but if you want to build a super high building or skyscraper dig down

If you have #mcpe 0.7.4, you can add servers following this Tutorial https://t.co/Fa6wcdJJsU
— PocketMine (@PocketMine) August 31, 2013

Ever since Mojang took down Pocket-Mine, their servers are still available, so here's a tutorial on how to add servers (Like LBSG) on 0.7.4

Tutorial: https://github.com/PocketMine/PocketMine-MP/wiki/How-to-add-servers

[NOTE]: The website is currently down and it will be back online again in a few hours.

~SpartanNova