I have misplaced my diamond impression mat and I need it for a cake tomorrow! Has anyone made their own lines? Do you have any advice?
If you mean something like a quilted pattern, I saw Buddy on CB do this:
Take a drafting triangle (those hard plastic ones - at Michael's in the art/painting section).
Get a tool like the Cutter/Embosser from Wilton. http://www.wilton.com/store/site/product.cfm?id=52538051-802D-F658-0A8270D4F8DBD7B6&killnav=1
Cover cake with fondant.
Measure and mark the size you need for the diamond. (i.e. 1 inch wide)
Place the triangle's shortest side on the cake board beside the cake, with the longer side being the side you will be working with.
Use the embosser tool and run it along the triangle to make an angled line. Move to the next mark, etc. Once done, flip the triangle over so you're going back the other way - repeat until it's all done.
I hope this is what you're looking for, and that it helps.
I know people who have used a ruler.
The first time I used my Diamond Impression Mat I didn't get very even results... so I took a thin dowel rod and gently went over the impressions. It really improved the effect, so maybe that's an option.... would be very similar to the ruler suggestion - so whatever you have on hand.