Every piece of stone (tesserae) used in creating the patterns and figural images in Roman mosaics were laid according to a set of rules (sometimes referred to as Andamento). To make a copy of a Roman mosaic you need to know these 9 rules and how they were applied. The best way is to learn in 3 stages.
1. Learn the Rules and their use in a ‘perfect’ mosaic. We are fortunate now that just by looking in books or on the internet we can learn how the Rules were used by seeing mosaics from all over the Roman Empire. At the time they were laid 2,000 years ago the craftsmen learnt just by an oral transmission, learning on the job so to speak. It is doubtful they had any manuals to work from so they would have learnt from their head/master mosaicist and the teaching would have been only as good as his work. So, from this we are able to learn almost a gold standard of the Rules, how they are applied in the best of work.
2. Learn the exceptions to the rules, a good example of this is not using triangles to disrupt a curved line but using keystone shaped tesserae instead. If we look at the standard double strand/plait Guilloche pattern, you can see keystone shapes used in the coloured lines but triangles are used in the inner black lines. This is because the angle of the black line is too acute and you would end up having to cut too many keystones.
In the original mosaics you’ll see that very few keystone cuts are used. When you use tesserae that are all slightly irregular you can just tease out the gaps to allow you to follow the curve without any other cutting.
3.Study the different standards of work, as with anything there will always be some workers you’d be glad to do your mosaic and some you wouldn’t, unless that was all you could afford. So when you look at original mosaics, see the Rules used, see the exceptions then try to determine the quality of the work. Some patterns are slightly off; some figures might be out of proportion (some of this may be stylistic but that is a different subject) and look at the backgrounds. The mosaicist could have done the figures then left his assistants to fill in the background, working at different angles you can get quite a disruptive effect. Normally when setting background you work on horizontal lines so look at a large background area and you can get an idea of which way the mosaicists wwere sitting as they worked.
Learn the Rules; apply them in your own work and whenever you get an opportunity look at the original mosaics to develop your eye.