I do not have a diamond impression mat and do not have the time to purchase one as I'd have to order one online.
Can this design be done without one? If so, how is this done? also, is the impression done AFTER the fondant is on the cake or before?
Yes, do the impression after the fondant is on the cake. I have used a 30-60-90 (or a 45-45-90) triangle (available at your local office supply store) to do diamon impressions. Just mark off evenly spaced dots around the bakse of your cake, place the ruler on, and use your tool of choice (quilting wheel) to mark off the sections using the slant of the triangle as your guide. Once you have done all the lines, flip the ruler over and do them the opposite way. HTH!