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: How do I unlock or upgrade Farmhands?
FarmVille 3: How does the Water Well work?
FarmVille 3: What is the Order Board?
FarmVille 3: What are Pets?
FarmVille 3: What is Happiness?
FarmVille 3: How do I make room on my farm?
FarmVille 3: How do I use the Barn Storage?
FarmVille 3: How do I get animals?
FarmVille 3: How do I use the Building and Decor Shops?
FarmVille 3: What are Achievements?