There are 3 ways to create a 'Roman' mosaic. You make a piece for piece copy of an original, next you use the main outline and highlights from an original, known as a line copy, finally you should be able to create the image or pattern totally freehand and then compose the mosaic following The Rules and in accordance with your standard of work.
The photo shows the first 2 methods. A piece for piece copy (bottom) allows you to be guided by the original mosaicists hand. The line copy (top) gives you the markers but the areas in between you have to set the pattern using your knowledge.
Piece for piece copies are great for getting a really good result from your mosaic but they are best for giving you the groundwork. What you don't get from them is speed, this you only develop from lines copies and freehand work.
When you create a Roman mosaic it's not just about applying The Rules, it's about how you work and this is something that can't be seen.