When I create a creature or character costume I usually do a bit of research on what I want to create. If its Iron Man then i look into the different kinds of armor there is and then I devise a means of creating it out of foam. If its a creature suit I make my own patterns and use a DTD "Duckt Tape Dummy" to build the costume over. With a professional gig it would be a fiberglass cast positive of the actor from a silicone mold. But the silicone mold and fiberglass casting usually costs a couple thousand. That is the primary reason for "DTD's" because Duckt tape is cheep and easy to come by.