You may get some idea reading the attached paper. It uses MoM to design unit cell. However, you could easily do the same in HFSS. Get the scattering parameter to calculate radiated power and plot those design cures to obtain the units in the array. If you have further quesries please let me know.
Yea, i'm also working on patch antenna with slots to get multiband behavior. But i don't know where to start. Please if anyone could post an HFSS example of patch antenna, it would help. Thks.