All you have to do is (X); that's enough—no more is required.
この書類に名前を書くだけでいいです。
All you need to do is write your name on this form.
朝ごはんはパンだけでいいよ。
Just bread is fine for breakfast.
手伝ってくれるだけでいいから来てください。
You don't have to do much—just coming to help is enough, please come.