There are 2 ways of doing this - However, You need to have at least 10 Farm Town neighbors to hire someone to plow but anyone can be hired to do the plowing and harvesting
1. You can plow and harvest other farms by going to the marketplace. To go to the marketplace click on "map" at the bottom right of the farm town screen. Select "marketplace". Other farmers will be there both to hire and to look for work. If a farmer hires you to work at their farm, a screen will pop up with a number of options - one of them is an option to be hired to work at their farm and the other is an option to decline. If you agree to do it for them, you will get the option to go with them to their farm. You will receive a notification to let you know if you are to harvest crops or plow fields. When you are done you select the option to "go home."
2. You can also go to a friends farm if they are active (a green light on their neighbour profile) and ask if they would like help with harvesting/plowing using the chat screen. If so they would need to click on your character, and select from the menu what they need you to do.
Previous Answer:
You cannot interact with other peoples farms on Farmtown.
You can visit their farms to look around, and sometimes to do special odd jobs (rake leaves, scare away crows, etc), but you cannot do the Main tasks of the game on their farms.
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Someone has to hire you to plow for them, and someone has to hire you as a harvester.
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