Creating a new town is one of the possible missions that lead to the rank of Alderman. You can accept the mission under the Alderman's Office menu in the Town Hall of the hometown of the current alderman.
You will have 1 year to build a town with all of it's facilities and make it to reach a population of 1000.
In order to build everything, You will need:
Around 1200 brick, 400 timber, 400 iron goods, 50 hemp and 200000 gold.
Large enough convoy(s) that can carry goods in this rate.
After you accept the mission, a new town (called a settlement) will be added to the map. You can't decide the place and the name of this settlement. When you visit it you will find an already constructed port, including the harbourmaster's watchtowers and the piers. You will have to build the different facilities in a given order:
Markethall: trade cranes will be added with it, so you can start to sell/buy goods to the town in a price of 0, so effectively it provides storage.
Trading Office: Gives you the usual abilities like building dwellings, businesses, warehouses and the ability to store goods separate from the market in your own office/warehouses.
Repair dock, roads, wells, Tavern
Buy/sell prices will remain 0 unless you finish the mission, so you don't have to build warehouses, you can use the marketplace for storage, but keep in mind that the citizens will consume the goods stored there but your businesses won't. Since sell prices will be 0 and you have to provide for all the needs of the citizens, you have to bring all the goods there for free until the settlement has a population of 1000.
As a life-time manager (effectively lord mayor), you don't need the permission of the council chamber, you can set the taxes to any values, build the next wall immediately after you finished one, and train as many units in the armory as many you want, but there is a limit of 20.
An easy way to increase the settlement's population is by dismissing sailors (sailors are hired beggars) in the town and then training them in the armory. Keep in mind that the town's guard has an upkeep cost. A population of 1000 can maintain around 10 units from normal taxes, so you have to donate gold to the coffers if you exceed this practical limit.
The mission will be done once you have the buildings constructed and the settlement reaches a population of 1000, you don't have to wait until the deadline.