Stretching your design to the full bleed guarantees that you won't end up with white edges/borders on your printed product. Here's how you stretch a design to the full bleed:

1. Click and drag the entire image to the top-left corner of the product.
   
   Note: When the image goes over the safety margin, it will automatically jump to the edge of the printing area, where it should be to cover the whole product.
   
   
2. Click the blue handle in the bottom-right corner of the image and drag it down and to the right until the whole printing area is covered by the image.
   
   Note: you may have to drag your mouse quite far past the product's edge to get the image to cover the whole printing area.