I used to make these all the time. What sized hand? The way works for both but its simpler when its bigger. Are these for an actor to wear?
I normally make them to wear, here are the steps and I will make you a Jpeg so you can see what Im talking about.
step#1 -heat form a piece of 1/4" plastic to curve around the forearm and extend past the hand
(the curved shape ,almost like half of a 3" pipe, adds support it straps to the forearm so the fingers are free to work the controls) The hand area flattens out (losing the curve) and then the end bends down 90degrees from the flat hand part.
Step#2- at the 90 degree bend strap hinges get attached with the barrel part (one side is flat the other side has the barrellish shape) at this stage they can bend either direction, use small bolts and nuts to attach them and predrill the holes.
Step #3- I use 1/2" Cpvc pipe for the finger bones (skin to taste), I heat bend those into a natural shape(slight bends at the knuckles), then attach them to the hinges, with an excess of an inch and a half overlapping the "hand" portion. The overlap and the attaching of the pipe will keep the hinge from bending backwards, now the fingers can only bend down.
Step#4- bungee cord is attached to the first knuckle of each finger and then attaches to the back of the hand portion a few inches behind the end of the pipe. Now, when you push the finger down it springs back up.
Step#5 - Remember the bend at the end of the palm? in that flat area under each finger drill a hole.
Step #6- now (on the finger) under the area where you attached the bungee cord attach a steel cable and run the other end through the hole within reach of the actors real fingers. then attach a ring to that end.
The actor pulls down with his fingers and that makes the steel cable pull the CPVC finger down, then the bungee returns it to the normal position.
This is a simple set up but is rugged enough to hold up to haunt use, alot of the finer fineky mechanisms wont last long in the hands of an actor. I will get the image up as soon as I make it, it may clarify if the details are fuzzy.