A recap of the evidence we have for guidelines used for a Roman mosaic. This copy I've made shows what was found at Villa Arianna, Stabiae, Italy (dated from 2nd BC to 1st AD). Areas of the tesserae have come away showing guidelines incised onto the nucleus, the layer that the setting bed is put onto.
A sharp tool incised the lines and a black charcoal wash paint was used to mark where the line of black tesserae goes. The whole floor was marked into squares then these bisected to create 4 triangles. Each part of the mosaic was then laid using some of these marker lines.
A good method as it is very easy for the mosaicist to follow. It's definite evidence of the Direct method in use.
Problems; it's slow, you trowel in just enough cement for one square at a time. There's no evidence to say how widespread this was, possibly an assistant being shown?
Also does this work on patterns like the guilloche borders (plait) where you have a lot of curves. One example shows just how one team of mosaicists worked, it cannot be taken as evidence for all.