I spent a morning researching the topic and discovered several recipes. Most Christian Churches have a recipe and they are happy to share them and the context in which they are developed.
The basic recipe is to use whole wheat flour and water. Mix it into a firm dough and roll out into a flat disc, thinner than your finger. Prick out with a fork to prevent the unleavened bread from blowing and bake at 210 degrees Celsius for about 15 minutes. The bread can be then broken into small pieces along the impressed lines.
Some more modern recipes use honey, salt, refining syrup and the like. There are also wheat flour wafers available, but I do not consider them as bread by my definition, though the ingredients are the same (It is the symbolic gesture of the Communion that is important).
The bread can be broken up when it is baked and cool to be used in Communion. An appropriate number off segments would be used and blessed by the priest, any blessed segments that are not used during the ceremony must be consumed by the priest after the communion.
From Medieval times, it is certain that monasteries abbeys and many churches would have had special arrangements to bake sacramental bread. Very few examples of such ovens survive today. I would like to be able to locate a few more examples.