This first card was made using the Sizzix small oval embossing diffuser which is pictured below. The idea of this diffuser is to have everything emboss on the card except what you have inside the oval cut out. As you can tell in the first card, the square lattice embossing folder embossed the card front minus the area where I heat embossed the greeting. It kept that area flat which I find is nice because it prevents your heat embossing from wanting to chip off after it's been run through an embossing folder.
The diffusers, which come in a 3 pack of an oval, a circle and a large center oval, have excellent directions in the packaging on how to use them. You simply put your paper in your embossing folder then layer the diffuser on top of the embossing folder according to where you want the blanked out area to be. You only need to use one plate though with the diffusers due to the thickness of them.
The diffusers also come with the cut out shapes by themselves, for example, the skinny oval, a larger oval and a circle. When these individual shapes are placed on top of your embossing folder you will get only the embossing inside the shape, not the entire piece of paper. These do just the opposite of what the above pictured diffuser does.
If you don't have the diffusers, you can always heat emboss your greeting on a separate piece of paper, punch it out and ink around the edges to make it stand out more on your card like in the example below: