Revise "telling" sentences into "showing" ones using concrete action and sensory detail, and combine choppy sentences using coordination and subordination.
The most common note on a first draft is "show, don't tell," and it means something specific: naming an emotion ("she was nervous") is weaker than rendering the physical evidence of that emotion ("her pen tapped twice against the desk before she pressed her palms flat to stop the shaking"). A precise verb often does this work by itself -- "she crept" shows more than "she walked quietly."
Showing usually produces more sentences, and more sentences risk sounding choppy. That is where sentence combining earns its keep: coordinating conjunctions (and, but, so) link two equal ideas, while subordinating conjunctions (because, although, when, since) rank one idea beneath another and show how they relate. "She was nervous. She dropped her pen." becomes "Because she was nervous, she dropped her pen" -- one relationship, clearly stated, instead of two flat facts sitting side by side.
Mastery looks like: They can revise a flat, telling sentence into a concrete, showing one, and correctly choose between coordination and subordination to combine two related ideas.
Common stumbles: Overcorrecting into purple, overwritten prose; combining sentences with "and" out of habit when a subordinating conjunction would show the actual relationship more precisely.
See it visually
Interactive, spoken-aloud tools that show this idea in motion.