BluePrint lets you map out your builds before creating them!
In order to craft a blueprint, you need a feather, paper and blue dye. Put them anywhere in a crafting table and you will get the blueprint.
You can build endlessly with the blueprint and if you are sneaking, the outline will turn red to map out windows and doors.
The outline vanishes if you far away so that on multiplayer, so that you don't see other players' blueprint sketches from far away.

Diamond is appreciated