The Farmer in Minecolonies is a very useful villager, able to harvest and vanilla crop and some modded ones too. First you have to build the Farmer his house and assign a villager to the job. Then, make a Scarecrow that will designate the field. Place this in the center of your field and right click on it. You will see s single inventory slot. Place the seed of the plant that you want to grow in here (it does not consume the seed). Then give the Farmer a hoe and seeds of the same type of the ones that you designated at the field. The Farmer can farm multiple fields depending on the level of its house.