How to Design a Nice Floor in Minecraft

First find 2 corresponding colors. (Black and White for example)

Then you lay out how you want your floor to be made.

Third, lay down your black item in every other square.

Fourth, lay your white item in the remaining holes. (That's what she said).

Lastly, place some torches, so that everyone can see your beautiful design.

How to Build a Mansion in Minecraft

You might want to install the Too Many Items mod for Minecraft for this.

First you need to figure out what you want your walls to be made of.  I recommend stone, since it is not a burnable object.

Then make a grid, the size that you want your mansion to be.

Third, you must make some rooms, and build a wall around your grid, so that you have a place to put a pool in.

Fourth, you must build the wall of the mansion and the rooms.

Lastly, put a roof that you like on it.