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 the Order Board?
FarmVille 3: What are Pets?
FarmVille 3: What is the Sky Race?
FarmVille 3: How does the Water Well work?
FarmVille 3: How do I breed animals?
FarmVille 3: How do I use the Barn Storage?
FarmVille 3: How do I see my Followers / Likers?
FarmVille 3: How do I expand my Farm?
FarmVille 3: How do I cook and craft?
FarmVille 3: How do Farmhand actions and supplies work?