you need lots of newspaper,
large bucket room temperature water , add PVA glue, you don't need much, buy it in bulk from DIY store or builders merchant.(massive can for the same price as a tube in craft shop)
stir in,
tear off thin short strips, through in one by one, to avoid clumping, just add as demand requires,
It is far better to do a few layers, don't do too many at one time as it works best if it totally dries out between sessions,
add a coat of PVA every now and then, finish in couples layers of white paper or the colour nearest to finish required, newspaper is a sod to paint over,
I asume you will be working off balloons, buy decent large ones, and don't blow up all the way, that way you get the size with less risk of popping before it's formed hard shell.