Ah, easy.
1. set you background color to the color of this website.
2. make the active layer the one you want to erase
3. load the basic brushes and select a soft lead mechanical brush with appropriate size.
4. select the eraser tool
5. click once and release in the top left corner.
6. hold down the shift button and click once in the top right corner, then lower right corner, then lower left corner, then top left corner.
holding down the shift will make it do a straight line between mouse clicks. You'll get the idea as you play with it a little, unless your wanting the more jagged edge look, then you can manually erase it until it look like how you want it.
Hope this helps.