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 Sky Race?
FarmVille 3: How do I use the Barn Storage?
FarmVille 3: How do I get animals?
FarmVille 3: What is the Market Stand?
FarmVille 3: How do I make room on my farm?
FarmVille 3: How do I cook and craft?
FarmVille 3: What is the Order Board?
FarmVille 3: How do I breed animals?
FarmVille 3: What are Achievements?
FarmVille 3: What is Happiness?