You can create your own farm by going into camp menu -> take an action -> create farm. A map icon will appear which will be called '<name>'s farm'. Go to your farm and interact with the workbench in the little shack. In fact there's a invisible guy. This guy stands on the workbench and makes you need to aim at a point higher to be able to interact with the workbench. Try this if you have issues with interaction.
Make sure you have 10 seeds and choose the option to plant these seeds at the field. Wait one week to harvest the crops. The wheat plants have a collision mesh that prevents you from walking through them. This is necessary to register a hit with your Farming Sickle. This tool is needed to be able to harvest the crops. Hit the crops to gain prestige, flour and sometimes seeds.
A farm takes 10 timber (buy or harvest from forest with permission from Lord who owns forest) and 1000 florins.
YOU CAN ONLY MAKE ONE FARM so place it somewhere convenient as you'll likely be returning there frequently.
Your farm comes with a storage chest, a work bench, and four fields.
At your work bench: You can choose to plant a field (takes 10 seeds), and after a time on the map you'll get a message saying you can harvest a field (harvest also decided at your work bench). There is also a quick farming guide at the work bench.
Harvesting: You harvest the grain in your field with a farmers sickle bought from the arms dealer in town. Choose which field to harvest from you work bench, equip your sickle and have at it. You'll get grain and one prestige point for each sheaf you harvest, and occasionally get some seeds as well so that you won't have to constantly buy them. The excess seeds can be replanted in an available field or stored in the storage chest. The grain can be sold or used as food for your army or new world colonies.
I currently do not know if you can hire others to work your farm for you or if you can add extra buildings or features to it. If anyone knows feel free to add it here and explain how it's done.