Use a square sheet of paper (you can trim an A4 sheet into a square), and start with something very simple like a «hat» or a «boat» that only needs 4-5 basic folds.
Fold the paper slowly in front of your child, step by step, and ask them to imitate you after each fold instead of finishing the shape and handing it to them. Participating in each step is what builds the skill, not just watching the result.
It's fine if the folds aren't perfect at first — the goal is to exercise your child's fingers in pressing, aligning, and precision. With repetition their fold accuracy improves, which positively carries over to writing and cutting skills later.
