Before you start
The checkerboard in the editor represents transparency; it is not part of your stamp. Use PNG to retain that transparency. A JPG copy will not preserve an alpha channel and can introduce an opaque background.
How to use it
Finish the design and inspect the text and border. Download the PNG directly rather than taking a screenshot of the preview. Add it to the image or PDF tool, position it over the content and check that the surrounding document remains visible.