Use the Designer:

  1. From the page where you enter your design's full size, click on the Draw button farther down the page.
  2. In the Designer, choose the white color and increase the line thickness. Then you can whiteout parts.
  3. When you're done, click the Print button.

If you'll want to crop your design, do that before erasing parts. Cropping is also accessible from the page where you enter the size.