Waves add together by superposition; that is, when two or more waves meet, the resultant is the algebraic sum of their displacements. Although this is a mathematical idea, this topic has many practical, hands-on activities once you begin to discuss the consequences of superposition (diffraction and interference effects). Since you will be working with light and sound, there are some great visual and audio effects.
You may come across two different approaches to this topic: a geometrical one and a conceptual one. The former starts with single slit diffraction, develops this to (Young’s) two slits and thence to multiple slit diffraction gratings. The latter first investigates superposition itself, followed by ‘interference fringes’ then diffraction.
Here we adopt the conceptual approach.