Step One.

First get the measurement you need for your arm. Measure from the shoulder to the wrist, I just doubled two full sheets of newspaper and used the size of the paper as the length of the arm. Best to double your newspaper. Gives you a sturdier arm to work with. Lightly roll the newspaper up into a 2 inch wide cylinder.

Apply duct tape as shown. The center piece is to mark the elbow. Now from the wrist end cut the cylinder in half up to the elbow and tape as shown in the next gif

This is now the completed armature for one arm.

Step Two

The hand.

What you'll be doing is making the finger and palm bones at the same time.

Needed things

Same things you used for the arm.

Tightly roll up some newspaper into long round strips.
Measure a hand from the wrist to the tips of the fingers and cut each piece of news paper to that measurement. I like to add an inch or more to each finger. Just looks cool. To make it easier I start off by holding the rolls of newspaper together with small strips of tape at each end and one in the middle. After getting the measurements for the bones I center and wrap a 1 inch piece of tape over each mark. This lets you cut through tape and paper so you end up with a bone with tape at both ends.

Position all five pieces for one hand as shown in next gif.

Starting with the palm use duct tape to hold 'bones' into place. For more detail start by taping the inside of the palm. Then on the outside press the tape down between each bone. You will end up with a bony hand. If the inside of the palm will be what is seen do the back of the hand first.

Referring to the next gifs for reference, tape the thumb at an angle.

Step Three

Positioning of the fingers.

Needed things

Lots of small strips of duct tape with a corner of each stuck to something so you can use one hand to hold the finger in place and one to get the tape.

Bend the finger. Tightly tape the bent area. It's that easy. May take some practice to get all the fingers bending in the same direction. But an oddly bent up hand looks cool when finished.

Before atatching hand to arm decide how your arm is to be positioned. If you need the elbow bent and how much. To bend and hold elbow into place use duct tape just like you did with the fingers. To keep the two smaller 'bones' in the forearm from flopping around tape the open end together.
For a bony elbow look wad a small piece of news paper up and tape it to the backside of the elbow.

Using duct tape attach the hand to the arm in the position you need it. Mache the entire arm and the back and inside of the palm. If your going to snot rag the hand you wont need to mache the thumbs or fingers.

f the arm is just going to hang down a second layer of mache is all you will need. However if the elbow is bent and or the arm is to be raised or at any angle it will need to be sturdier. The more layers of paper mache now the stronger the arms and the longer your prop will last. I like using 1 X 1 1/2 ripped pieces of newspaper for mache. Overlaping each piece and going in four different directions for each layer. 4 or 5 complete layers of paper mache will make the elbows sturdy.

1 complete layer of paper mache =

First all pieces of paper overlapping and going in the same directon.

Second all pieces of paper overlapping and going in a 90 degree angle from the first layer.

Third all pieces of paper overlapping and going in a 45 degree angle form the second layer

Fourth all pieces of paper overlapping and going in a 90 degree angle from the third layer

This is one layer of paper mache. Let dry before applying a second layer.

Attaching the arm to the shoulder should be treated the same. If it's just going to hang down or the corpse is going to be permanently part of a coffin than 2 or 3 layers of mache will work. If the arm is raised then the mache will have to be strong enough to hold the weight. 5 or 6 layers on the shoulder, up to the neck and at least 4 inches around the shoulder. You only have this chance to make it strong.