Do you mean how do they make them? There were some breeds that were already small such as d'uccles and Japanese, they are considered true bantams, you would have to look them up for more info. To get a largefowl small you could either select for smaller and smaller birds over time or you could cross to something similar that is already a bantam and go from there.
Lets use Marans as an example, I am going to go by the type standard mainly. Since there are already barred rock bantams you could cross a barred rock with a cuckoo maran. Keep only the ones with white legs and the most maran characteristics like loose feathering, white legs, cuckoo instead of barred. Then you could either breed those together to get more that are inbetween sized and all or you could breed the most maran looking ones back to a barred rock to get the size down and all that. I am sure there are other ways but thats one easy way.