I would first start by introducing decimals and what they are. That it separates whole numbers into parts. One great way to do this is using real examples like money. Than its important to teach them the place values. Visual hands on ways like a chart or blocks can be teaching tools to help a class understand. Decimal numbers are related to rational and irrational numbers because decimals can be both. Repeating decimals can be rational numbers where non repeating, non terminating decimals are irrational numbers