Instructions
Buy one bag of Grass Seed for 600G from Jean's shop. Buy more bags of Grass Seed if you can afford it. Clear the land of weeds and herbs. Sell the weeds and herbs, throw them away or consume the herbs for stamina.
Equip your hoe and dig a 3x3 plot. Walk to the middle square of the 3x3 plot and use the bag of Grass Seed. All nine tilled squares should have seeds. Equip your watering can and water the 3x3 plot.
Pick any herbs or weeds that sprout around the plot of grass. If you don't pick the herbs or weeds, it could be more difficult for you to water and harvest your grass. Water the 3x3 plot every day for nine days. On the 9th day, the grass should be fully-grown.
Equip your sickle and stand in front of a square of grass. Press "B" to use the sickle to harvest the grass. The grass should go into the feed bins of your monster huts instead of your inventory. You should also get one Rune Point. The amount of grass you harvest depends on the amount of monster huts you have.
Continue to water the grass after harvesting the fodder. Just like in the real world, the grass will grow as long as you water it every day. Your grass will regrow every two days.