Typically I use 1 part portland, 1 part peatmoss ( or potting soild with peatmoss as the main ingredient, and 1 part perlite.
If the potting soil is main of peatmoss - then it is ok.
Next mix all above with water until the consistency of raw hamburger. That sounds gross, but it is not too wet but throughtly moist. If your mix is too dry, you may have a crumbly mix.
Start again. mix what you have with water and then place in the mold. COver with garbage bag - seal tight.
Allow to cure for about 24 to 30 hrs. remove from mold, rough up the hypertufa ( if neded) then hose down with water and put back in bag for 1-2 weeks.

Here is a link that might be useful: link to one of my tufa photos