Making a hand of ice is pretty easy: fill a glove with water and put it in the freezer. That said, the following tips will help.
My advice is to use a flimsy dishwashing glove. You can, of course, use a latex glove (as for medical exams) or nitrate glove (as for automotive work), but the drawback for these is that lefts and rights are interchangeable; they look less like a real hand because the thumb is positioned improperly.
Suspending it in the freezer
You’ll need a way to suspend your glove in the freezer while the water crystallizes. I recommend driving a pair of skewers (as for shish kebabs) through the glove from perpendicular angles about an inch or so below the wrist opening. Then you can hang your glove inside an empty oatmeal box (you know, the round sort).
Water expands as it freezes, which can give your frozen hand and unnatural degree of swelling in the palm, looking quite as though it had suffered a number of bee stings. To minimize this, add just a bit of water at a time to your glove, wait for it to freeze, add more water, etc.
Where to stop
Don’t fill the glove further than the base of the hand. An ordinary wrist is narrower than the hand, but dishwashing gloves are not that shape.
Protecting the fingers
When you remove your dishwashing glove from the frozen hand, the fingers are liable to snap off. That’s okay. Apply a bit of water to the break, restore the severed finger to its place, hold it so for a couple of seconds, and put it back in the freezer for a little while.
My advice is to not make your ice hand far in advance. Modern domestic freezers are all ‘Frost free’, which means that they periodically (all too frequently) go through periods of thawing and freezing. This will wreak havoc on your hand of ice. In the end, my hand’s fingers became shortened and clawlike.
… And finally
Don’t forget to put Erasmus’ key into the glove before you fill it with water.