first you want to create a new world once your done with make a super flat world now you will see a whole bunch of crap what you want to do is go to http://media-mcw.cursecdn.com/8/8c/DataValuesBeta.png and look at the dat values and the blocks once that happens go to your superflat world once that is done click CUSTOMIZE now once that happens you will see the main numbers on the top bar with a village on the end now the village doesnt do anything ok so now you see the main numbers which are 7, 2, 3 now 7 is bedrock and 3 and 2 are just grass and dirt now what you want to do is you delete the block that will be replaced and replace it with the block id of what you want for more info checkout the video