In most cases, exactly like loaves of bread made today in various parts of the world. Non-mass-produced breadmaking hasn't changed much.
Ancient Egyptian tomb paintings show a bread identical to that still made in Luxor, Egypt, called shamsi bread by modern Egyptians, and "sun bread" in English. They were round loaves with various designs picked out on the top. [1][2]
Ancient Roman bread-making is well documented in literature, and at least one carbonized bread loaf has been found in the ruins of Pompeii. It was a round loaf, segmented in pie-like wedges, with the baker's stamp still visible. (Roman bakers used bronze stamps to embed identifying marks in the bread loaves; by Roman law, all bread sold had identify who baked it, so if it was adulterated or underweight, the offending baker could be found and prosecuted).[3]
There are a lot of resources on the web for ancient and medieval bread-making; it seems to be a popular topic. Possibly even more popular than pre-modern beer-making...
[1] [Theban Mapping Project, "Sun Bread of Luxor"] (http://www.thebanmappingproject.com/articles/article_15.4.html)
[2] Egyptian Sun Bread
[3] Pompeiian bread loaf & a recipe
Hey, not discouraging further answers, though you might find these previous threads a good starting point while you wait:
If I were to eat 3 loaves of bread, one from Egypt in the year 2200BC, one from Rome in the year 45BC, and one from England in the year 1100AD, how much different would each one be to the modern day and/or to each other? by /u/alriclofgar and /u/IM1RU1too
AskHistorians: What was the bread (or bread analogue!) like in your time period? by /u/snackburrows and /u/Giesskane
If you're also interested in other time periods and/or the process of making bread, check these out:
Was bread made in medieval times the same way it is made today or has the ingredients of bread changed throughout history? by /u/agentdcf
The AskHistorians Podcast 063 - Milling and Baking in 19th Century Britain
The AskHistorians Podcast 064 - Milling and Baking in 19th Century Britain, Part 2