Start when the low temps are above 50 degree. If the temps go below 50, the cement ( portland) will not cure properly.
I am in PA and have similar low temps. Still waiting to start
Suggestion - Start with a small project. Go get a large snack bowl and use that for mold. Mix small batches, and measure your ingredients, with a container like a coffee can. Any of the formulas work - Mine is simple
1pt portland, one peat ( or potting soil) 1 perlite.
Store your Portland in a sealed container. That will help to keep moisture from getting to portland cement. I use Kitty litter containers - About 1/3 of 90lb bag fits and lids snap off and on easily.

{{gwi:68271}}

Now -- Just Do It !

Here is a link that might be useful: Garden Design Ideas

You can do this and you will have so much fun creating. Mostly everything works. I've had a few failures but the results have far out weighed the mistakes. If you have questions just ask. Have Fun.

Thanks for the advice and encouragement! Nighttime temps are forecast to be above 50 for the next 10 nights. The moment could be at hand!

I've been known to drag some things in the house when I just can't wait!