This is really a very simple movement. Get a big bucket and fill it with rice. I've also seen rubber chips, metal shot and sand used. The metal shot is hardcore and should only be used by those with a very strong grip to begin with. Rice is your best bet as it's the easiest to use and you can always progress up to the sand from there.
I made mine with a big laundry tub I found at the dollar store and a huge bag of rice from Sam’s club. Once you get it set up all you do is dig your hands in and begin squeezing. Pretty simple stuff, nothing too complicated.
I've also found I get great results when I twist my wrists both clockwise and counterclockwise as I squeeze. This is best done to failure at the end of your training session, so get digging!
