Summary: Start by speaking to Farmer Gricoller (located in the Hosidius House in the Great Kourend) Pick up some seeds off the table and go to the plots and begin planting your seeds. Water them every growth tick, then harvest. Takes approximately 3 minutes for plants to fully grow. Once harvested deposit fruit into the deposit sacks and recieve xp bonus.
Required stats: 34 farming (bare minimum, The higher the level the higher the xp/hr)
Required quests: 100% Favour in Hosidius House
Required items: Minimum of 6 watering cans(8), seed dibber, spade.
Other notes: If this is done right you can gain somewhere around 100k xp/hr at 74 farming. This is by far the easiest way to gain farming xp while having absolutely no inputs so its basically free farming xp. As a plus to this "minigame" you can achieve the Farmer's Outfit once enough points are gained, the outfit gives a 2.5% experience bonus and only costs 400 points to complete.