I was wonder ing how to get the diamond momogram look and how to get the lines perfectly straight.

this is what I want it to look like...

http://1.bp.blogspot.com/__aBmLkwV3Wc/TQ9j7ZsRI/AAAAAAAAAdQ/8-2ZJClDJ9A/s1600/Quilted%2BIvory%2Band%2BBlack%2BWedding%2BCake.jpg

Thank you : )

The easiest was is a diamond impression mat sold at most cake supply stores.
HTH

There is an impression mat for this, I think it's the easiest way to achieve straight lines, then just run a quilting tool over the impression lines.

thank you! Can this only work on Fondant or does it also work on buttercream?

both.