I use a five gallon bucket.
** 2.5 buckets wet, shredded paper
** .75 buckets sand
** .25 buckets Portland cement
** some boric acid (lately I put a whole bottle of Roach Pruf)
** two, maybe three buckets water
The longer the paper soaks, the better.
For sculpture, I mix it by hand in a five gallon bucket.
I use cellulose insulation, which is simply pulped paper, purchased from Lowes for $10 for...I don't know...a big bag, and a large (2-pound) coffee can as a measuring device.
** 4 cans cellulose insulation
** 2 cans sand
** 1 can cement
** 2 to 2.5 cans water -- enough to mix it well but not make it too runny
** plenty of premixed joint compound
I just mix it by (rubber gloved) hand in a 5-gallon bucket. I mix everything but the joint compound, and add that just before use--a couple of big handfuls. I find I sometimes have to add more joint compound as I work, depending on the project at hand, and keep a bucket of water nearby to wash the joint compound off my gloves so the crete won't stick to them.
It's just a starting place.
My formulas are always evolving, and yours probably will too as you work with this amazing stuff.