Tap on an empty plot, then drag the crop of your choice over the empty plots.
Wait for it to grow fully or use a Water Boost (unlocks at level 8) to speed up its growth.
Tap the crops that you want to harvest, then drag the sickle over the crops to reap them.
Related posts:
FarmVille 3: What is Happiness?
FarmVille 3: How do I expand my Farm?
FarmVille 3: How do I change the appearance of my buildings?
FarmVille 3: How does the Water Well work?
FarmVille 3: What are Achievements?
FarmVille 3: What is the Order Board?
FarmVille 3: What are Decoration Coins and how do I earn more?
FarmVille 3: How do Farmhand actions and supplies work?
FarmVille 3: How do I use the Barn Storage?
FarmVille 3: What is the Market Stand?