I'll teach you how to use the third-party world-editing tool MCEdit! It can be found at its official site at mcedit.net. First, you will need a copy of Minecraft and MCEdit. I'll be using the 2.0 version, but you can use any version. After running

The selection tool can also be used to export schematics and do other basic tasks. Some tools cannot be used without a selection. Brush Tool. The brush tool allows you to "Paint" the world using preset shapes, you can use RFEG to Increase size,

If you're using the old version of MCEdit, try installing the 1.8/1.9 version. If you have MCEdit 2, then you have the wrong kind of MCEdit. It should work fine up to 1.11 but I can't guarantee.