We started with army helmets and hockey masks, built up the mask skeletons with foam (used the foam from flooring tiles you might use on a garage floor) before covering with Crayola Model Magic, decoupage gloss and spray paint. The Berserker mask was covered with truck bed covering spray (which made it really durable). We drilled holes in the back side of the helmet and inserted the dreads with wire (about 30 or so per helmet). We inserted a laser on the side of the helmets that can be turned on by pressing a button (much like the Predators had in the movie).
Dreads were made from light-weight caulk saver foam insulation wrapped in either black athletic tape or friction tape (you can get this at any hardware store). Once inserted in the helmet, we covered the tops of the dreads with felt to hide the wire and create tubes coming out of the head. Covered the eye holes with black mesh and cushioned the inside of the helmet with upholstery foam to keep the helmet snug on their head.
Latex skin was made by using an existing Hulk Hogan muscle costume with about 3 layers of liquid latex mixed with acrylic paint to cover (use a well-ventilated place, golves and a ventilator if you plan to use liquid latex), followed by airbrushed Predator details (we cut out templates for details). Before latexing the suits, we made tape dummies (wrapped kids up with duct tape, cut them off, retaped them and stuffed them) to keep the latex from shrinking too small. Instead the latex formed to their exact body size.
Armor was made out of additional foam floor mats, zip-ties and old belts and wood for swords. We had netting (used pair of fish-net hose) for the classic Predator, but our son opted to not wear it.