You can make many different mixes, there is no RIGHT mix, but this is one that I really like
50-50 portland cement and vermiculite.
You can also do a mix that is 1/3 portland, 1/3 peat moss, and 1/3 vermiculite.
I really like the vermiculite because it adds a nice fawn color, sparkle, and a cool texture.
You could also use perlite.
The bird bath is made with a plastic oil pan that you can get at an automotive store. You spray it with non-stick cooking spray and the add your hypertufa mix, gradually patting it down as you go to avoid air pockets. I mix my hypertufa in a BIG mixing bowl from the dollar store.
This is an excellent link for projects, recipes, and info about hypertufa: