It's not known for certain how it was done and we've never found a sketch as far as I know, but some form of master plan is generally suspected for figure mosaics. The customers purchasing the mosaics to be constructed would have selected the images and motifs depicted beforehand and would have lacked the skills of the artists to imagine them in tile. Some of the fancier mosaics also involved custom tiles that had to be manufactured beforehand, and knowing how many are required is valuable.

It seems likely that some simpler mosaics and patterns were done straight from the artist's mind though.