The temp is rather important - There needs to be a constant 50 degrees so that the tufa mix will cure properly. So if it gets colder, there is a chance that the project will not cure long enough -- I am sure some of us have done project when it was cold and they worked - but try to keep the location of the finished product as close to 50 as possible . The first time I tried in my garage in winter - many of my projects cracked. I am not sure it resulted becasue of the cold or it was my first time.

Start small - and bring inside where it is warmer to cure ( needs to stay damp for a few weeks - so wrap in garbage bag and mist with water .)
Go to Home Depot or Lowes and ask them for Portland Cement -that is the product name - the manufacuter will vary - Quickcrete and Lafarge is the manufacuter. Most cases - portland cement comes in 90lb bag. There are many types I and II, III ( you can google to find the real difference) all are slightly different and mainly sold that way for concrete contractors. I beleive any of those will work - I bet if you go to one of the big box stores you will find what you need. BTW -- do not buy concrete
Any sand will work - however I dont always use it. Sand just makes it a little heavier.

I use peatmoss (or just some cheap potting soil)
and also add some perlite. Usually use 1 part each (Portland, Peat, perlite) - and water ! thats it ------

The best part of hypertufa is that there are many recipes Try them all! LOL!
Mike

Here is a link that might be useful: My Blog

Thanks for the info, Mike.
This forum is great for learning. I know I will learn alot as I try it and experiment, but it never hurts to get as much info as you can before you begin. If I can't manage this project during the winter, I probably can't fit it into my schedule. Come Spring, I have tons of winter sown plants to play with and that keeps me busy.
So I will research as much as I can, then give it a shot, making plenty of notes along the way and stopping in this forum for advice and reporting my progress
Thanks again!
susan