Start by drawing a rectangle in the exact dimensions of your envelope. This will be used to locate your addresses. I made it white just so I could see what my text looked like a bit better.
Find a writing font you love and add an address of your choice. You will need to add a from and a to address.
Then use your rectangle to locate both addresses correctly. You will also want to make sure that your writing font is set to draw lines.
Delete the rectangle and attach the two addressed together.
Then click make it to see it on your mat. Do note here that your design is moved 1/4 inch in from both edges when drawing.