A fat suit can be made by stuffing an oversized leotard with polystyrene beads and micro-beads and attaching a latex breastplate that is also stuffed with polystyrene beads. The stuffed pockets of "fat" are contained within a polyester lightweight knit, replicating the skin. This design is based on the fat suit worn by Drew Frady in the musical comedy "Hairspray."
It will also help to research the ways in which the body holds fat by looking at medical textbooks. The hips, buttocks and breasts should be emphasized with extra stuffing.
If the fat suit is likely to be used multiple times, it is a good idea to make the fat pockets detachable for washing.