what is the initial step in designing multi band rectangular patch antenna?

i read the books, but of course, its only explain the step of making simple design of patch antenna with only operate in one operating frequency
i need to design a multi band antenna (dual band) working in wimax f band for 2.5 and 3.5 Ghz

how to start my design for multi band antenna?

i really hope you guys can give me , an ignition to start the multi band design
cos i know nth about it
TQ in advance