I've seen this done with a full (bottom) layer of fondant, then the diamonds set on top. For my first attempt, I would consider doing a very light guideline in the bottom fondant where each diamond touches so you can lay them perfectly. Use a ruler and an embosser. Not sure it would work, but worth a try on a practice piece.
To make them flush, you will need to use a diamond shaped cookie cutter to cut out the fondant pieces, and then place them on the cake in the same patter as shown on the cake. Kind of like a puzzle.
Quote by @%username% on %date%
%body%