They're pointy. Therefore, for the sake of safety, make the blades out of craft-foam. Draw out the pattern on a big piece of paper. Cut out the shape on the paper. It's probably easier to cut them separately than to try to stack them up and cut at once. Tack the paper to the foam sheet and carefully cut it out.
The trickier part is gonna be mounting them to your arms. Many techniques would cause them to flop all over the place, and possibly tear off your arm. I'd try something like buying sheet polystyrene from the hobby shop, heat forming it to wrap around your arm. Cutting it to match those blue tattoo-blotch thingies. File all the edges round after cutting out the shape. Then cut out some wooden dowel pieces, rasp a curve matching the curve of the arm-blotch, pre-drill a hole through the curved end, predrill a hole through the blotchy thing, coat the dowel end in wood glue, and screw it in place. Let the glue dry for a day or so before touching it. You might need to tie/temporary-tape the thing down do a dummy arm so the plastic holds the proper curve while the glue dries. To get the blade on the dowel, saw a notch into the other end, about 1/2''. Rasp the end of the dowel to have a more natural shape, and glue the foam blade into the notch with wood glue.
To get the arm blotches to stay in place, use something like Velcro, or high-test fishing line, firmly attached at points along the piece. If this is still not enough, you can further secure it with pieces of Hollywood tape on the underside.