Hold a tool, like a shovel or hoe, and type //brush followed by the type of brush and properties. After you do this, the tool will act as that brush. Click on a block with the tool to activate.
Example:
//brush tree jungle
When you click this tool on the ground, a jungle tree will appear in that spot.
Two really good brushes:
//brush sphere [block-type] [radius]
If you add -h, the sphere will be hollow. For example:
//brush sphere -h glass 20
//brush cyl [block-type] [radius] {height, optional}
Creates a cylinder or circle. If you leave off the height, it defaults to one block high, so basically just a circle.
Can also add -h here to create a hollow circle.