Experiment Design: variables and controls

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I'm learning "Experiment Design: variables and controls" on The Future Market. Lesson 1: "From a question to a written design".

What the lesson teaches:
- A good design starts with a testable question ("Does ___ affect ___?") and a hypothesis with "If... then... because..." (course 15.9, lesson 2).
- Name your independent variable (what you change), dependent variable (what you measure) and controlled variables (what you keep the same).
- A confounding variable is something else that changes along with your independent variable and could explain your result. Example: you test light on bean plants but the plant in the sunny window is also warmer. Light and warmth are mixed up, so you cannot tell which one caused the change.
- An operational definition says exactly how you will measure something. "Grows well" is vague. "Height in centimetres from soil to the highest leaf tip after 10 days" is operational.
- Choose levels of the independent variable: at least three levels (for example 10 °C, 20 °C and 40 °C) let you see a pattern, while two only show a difference.
- A written plan lists: question, hypothesis, variables, materials, numbered procedure, how data will be recorded, safety notes and how many trials.

The exercise I'm working on:
Take your best testable question. Write the independent variable with three levels, the dependent variable with an operational definition, five controlled variables, and one possible confounding variable with a plan to stop it. ---

I'm stuck. Please:
1. Explain the main idea in plain, simple words with one everyday example.
2. Walk me through the exercise one small step at a time. Ask me what I've done so far before giving the next step.
3. Don't just give me the final answer. Help me do it myself, and check my work when I share it.

I'm a parent helping my child (about 8 years old) with a lesson called "From a question to a written design" from the course "Experiment Design: variables and controls".

Here's what the lesson says:
Before you bake a cake you read the recipe. An experiment plan is the recipe: what you will change, what you will measure, what you will keep the same, and what you will do, step by step, so anyone could repeat it. Before this course: The Scientific Method & Science Fair Projects (course 15.9). You already know the steps of the scientific method and the three kinds of variables. Here we go one step deeper: design controlled experiments that stand up to a judge's questions.

The activity:
Take your best testable question. Write the independent variable with three levels, the dependent variable with an operational definition, five controlled variables, and one possible confounding variable with a plan to stop it. ---

Please explain this so an 8-year-old understands: short sentences, a fun everyday example, and no hard words.
Then give me 3 simple questions I can ask my child to check they understood, and a way to do the activity together, one tiny step at a time.