Once you have enough prisoners, go to a castle, select request entry to the keep, go to the dungeon and select ' Donate Prisoners'.
When you click the button, the selected lord will be killed immediately. A pop-up window will appear in which you must confirm your decision by selecting the Execute Him option. To perform an execution, you must click on the name of the selected lord and then select the red icon called Execute Prisoner.
Once you have taken prisoners, you may recruit them to your party, ransom them to Ransom Brokers (found in taverns in towns), or garrison them in a town or city. What do you do with prisoners in Mount and Blade Warband? Then you go down in middle-south and you can sell them for 200-300 each :D. Usually you get them around 100 gold each.
So I go in the snowy top region of the map where there are a lot of villages with cows (like 5 + the cities which sell cows).