Corrected sentence: Squirrels make stashes of mushrooms, nuts, and acorns for the winter.
Simple alternative: Squirrels store mushrooms, nuts, and acorns for the winter.
Why this is better (easy steps):
- Who and what: "Squirrels" is who, and "make" or "store" is what they do.
- Commas: Put a comma after each item in a list. We put a comma after "nuts" before "and" (this is called the Oxford comma).
- "the winter": Adding "the" makes the time clearer: "for the winter."
- Word choice: "store" is a simpler word than "make stashes," but both are correct.
So your sentence was almost perfect—just add a comma and "the" to make it extra clear.