There are several ideas for costumes available from sources like cartoons, video games, or movies. Some of the more fantastic creatures you'll want to replicate as a costume may have more than two arms. In these instances, you'll need to fabricate an extra set of arms to complete the costume's illusion. Luckily, this can be done relatively easily. The extra arms won't be able to move on their own, but they will add some neat visual flair.
Things You'll Need
Long sleeve t-shirt
Pair of gloves
Cut the sleeves off a t-shirt.
Stuff the sleeves and both gloves with newspaper or soft stuffing from a craft store. The stuffing will be quieter and more movable, but newspaper is typically more readily available. Either one is fine.
Sew one glove onto the end of each sleeve.
Sew the gloveless ends of the sleeves to the main body of the costume, underneath the costume's main arms and on the sides. This is the typical orientation of most four-armed creatures.
Sew one end of a length of string to the top of the false arm. Sew the other end into the bottom of the costume's main arm, directly above the false arm. Sew on a couple more lengths of string to ensure a sturdy connection to the main arm. Now when the main arm is moved, the false arms will move with it.