Using the Clipping Tool

The clipping tool can be found on the left hand side of the screen, its the brown wood style block with a yellow cut out bit on.

Select your object and click on the icon, now you'll get some white lines and 2 blue dots, just move it around and have a play with it, when you think you've got what you want press return and it will perform the clipping, its useful to clip corners of blocks to make them rounded. You can also make all sorts of shapes with this tool, its very handy indeed, but dont go mad with it, try to keep the shapes simple, dont go clipping every side of the brush.