Designing a multiband antenna often starts with a solid single-band structure. For example, a basic PIFA tuned to the lowest desired frequency. From there, additional resonances can be introduced by adding slots or notches to the radiating element or ground plane. 

Each slot behaves like a parasitic resonator, so the antenna can support more than one frequency band without drastically increasing its size. 

The key is to control the slot dimensions and positions to target specific bands while maintaining acceptable matching and isolation.