For the diagonal holding, I made a 1-2 twist of the yellow silk. One strand of silk is divided into four; two of the quarters are under-twisted separately before being over-twisted together. This makes a very fine, strong thread which is laid across the foundation. If the laid stitches are at the wrong angle, the thread is clearly visible.
The trick is to angle the holding thread across the foundation in such a way that it merges with the twist in the foundation threads.
Finally, the laid stitches are couched with the same thread.