Instructions
Look for Joan wandering around the town on Sunday. Joan is a boar with a large pack on her back.
Speak to Joan and purchase a red turnip seed packet for 1,000 bells. You're only able to buy one packet per week.
Plant the turnip seeds in the ground by your house. You don't need to dig a hole to plant the seeds. Simply select the pack in your inventory and select "Plant."
Water the turnips every day of the week. If you miss a day of watering, the turnips will rot, and you won't be able to sell them.
Equip your shovel on Saturday and dig the turnips out of the ground. Go to Tom Nook's store and offer the turnips for sale. Tom Nook will pay you 16,000 bells for them, so you will make a profit of 15,000 bells.