The clipper tool is useful for making shaped objects. Its located on the top bar, here:



Now lets make an obelisk. An obelisk is an ancient Egyptian type of structure, I chose it because its very easy to make.



Select the brush and hit the clipper button. I suggest hitting the XYZ button to the right of the clipper tool to change the 2d grid view. Now lets start clipping! To make a clip, select two points of the brush by clicking. Like so:



Then hit SHIFT-ENTER to make a clip. Now youve got a nicely cut brush.



Deselect everything and delete the extra brush and do the same to the other side until it looks like this:



Its coming along. Now switch the XYZ view one more time and do the same cutting. You should now have an obelisk!



Ignore the caulk texture, that happens when you cut things. First click the clipper tool button to turn it off. Then select the whole brush and add the texture back on to it. Voila, an obelisk!